AI Slack Emoji Generator: Prompts, Tools and the 128 KB Rule

An AI Slack emoji generator is any AI image tool plus one extra step: clear the picture's background and crop it tight around the subject, then upload it with Add Emoji in Slack's emoji menu. The AI part takes seconds in ChatGPT, Gemini or a dedicated emoji site. File size is not the problem for a still image: in our test Slack accepted a 3.6 MB PNG and shrank it to 128 x 128 itself. Most AI emoji fail because the tools return a small subject on a white background, which looks like a smudge at 22 pixels.

We make the free Slack emoji maker, which takes the image your AI tool made, clears the background, crops it square and saves a file Slack accepts. It runs in your browser and does not upload the picture.

How to make a Slack emoji with AI

• Write a prompt for a tiny picture. One subject, a thick outline, flat colors and a plain background. The prompts below are a starting point.
• Generate it. Any AI image tool works: ChatGPT, Gemini, or one of the emoji sites in the next section. Ask for a square image.
• Make it fit Slack. Slack's help article "Add custom emoji and aliases to your workspace" says square images under 128 KB with transparent backgrounds work best, in JPG, PNG or GIF, and GIFs can have up to 50 frames. Drop the image into the Slack emoji maker, tick Remove background, and download the 128 x 128 PNG. If you only need to shrink a file, the Slack emoji resizer does that.
• Upload it. On desktop, click the smiley face in the message field, click Add Emoji, then Upload Image, give it a name and click Save. The full steps, with admin permissions, are in how to add custom emoji to Slack.

What Slack does with a large AI image, tested

AI tools return big files, often 1024 x 1024 pixels and several megabytes. We uploaded two test images to our own Slack workspace on October 4, 2026 through the same emoji.add call the Add Emoji dialog uses, then downloaded what Slack stored:

upload                                         answer   stored by Slack
1024 x 1024 PNG, 3,608,896 bytes (noise)       ok       128 x 128 PNG, 40,977 bytes
1200 x 600 PNG, 16,485 bytes (wide)            ok       128 x 128 PNG, transparent bands above and below

So for a still image you do not need to resize or compress anything: Slack scales it to 128 x 128. It also pads a wide picture with transparent space instead of cropping it, which makes the subject smaller, so crop it square yourself. The 128 KB limit matters for animated GIFs, which have to be under 128 KB after Slack's resize; our emoji-pack tests on September 30 got resized_but_still_too_large for GIFs over that. We deleted both test emoji afterwards.

AI emoji generators that rank for Slack

We checked the sites in the top search results for "ai slack emoji generator" on 28 September. Prices are what each site's page said that day.

Tool	Cost	What to check before upload
Pixelle (pixelle.io)	Free to start, no card	It tells you to generate at 128 x 128; check the file size
Forgemoji	Free	Outputs a 256 x 256 transparent PNG; its page lists a 1 MB limit, Slack's help says under 128 KB
EmoteMaker.ai	Credits: $5 for 25, a static emote costs 5 to 7	Says it sizes emotes for Slack
Venngage	Free, with an account	Resize and clear the background
ChatGPT, Gemini	Your plan	Large images, often on white: resize and clear the background

A dedicated emoji site saves you the resize step. A general AI image tool gives you more control over the drawing. Either way, look at the result at 22 pixels before you upload it: that is about the size of a reaction in a busy channel.

Prompts for AI Slack emoji

Emoji are read at 16 to 32 pixels, so a prompt has to ask for less than you think. These are ours; copy them and change the subject.

A single [subject] emoji, flat vector style, thick dark outline,
2-3 bright colors, centered, fills the frame, plain white background,
no text, no shadow, readable at 32 pixels, square image

A cute [animal] face emoji giving a thumbs up, kawaii style,
thick outline, solid flat colors, plain white background, square

Pixel art [object] emoji, 32x32 pixel grid, bold outline,
limited palette, plain white background, square

• • One subject. A dog, a rocket, a coffee cup. Scenes turn into noise at 22 pixels.
• • Thick outline and flat color. Thin lines and gradients vanish when Slack scales the image down.
• • Plain background. A single color is easy to clear; a busy background is not.
• • No words, or one short word. Letters under about 8 pixels tall do not read. For a text emoji such as "LGTM", the Text to emoji tab of the emoji maker gives sharper letters than an AI drawing.

If you need ideas for what to make, see Slack emoji ideas and Slack emoji packs.

Make the AI image Slack-ready in the browser

The screenshot is the Slack emoji maker with its sample image. After you drop in the AI image it shows the output size, checks Under 128 KB and Square, and previews the emoji at 16, 22 and 32 pixels on light and dark themes. Remove background clears the color around the edges, which suits the plain backgrounds AI tools draw. You can also add an animated effect such as party or intensifies; the tool shows the file size of each one before you pick it. Starting from a real photo instead of an AI drawing? See how to make a Slack emoji from a picture.

For more on transparent backgrounds, see how to make an emoji background transparent for Slack.

Make it Slack-ready with a script

If you make many AI emoji, a short Python script with Pillow does the same job. It makes near-white pixels transparent, crops to the drawing, pads to a square and saves a 128 x 128 PNG:

from PIL import Image, ImageChops
import os, sys

def slack_ready(src, dst, size=128, white_cutoff=245):
    """Turn a large AI image on a white background into a Slack emoji PNG."""
    img = Image.open(src).convert("RGBA")
    # Make near-white pixels transparent: most AI emoji come on white.
    keep = img.convert("L").point(lambda v: 0 if v >= white_cutoff else 255)
    img.putalpha(ImageChops.multiply(img.getchannel("A"), keep))
    # Crop to the drawing, then pad to a square so nothing is stretched.
    img = img.crop(img.getbbox())
    side = max(img.size)
    square = Image.new("RGBA", (side, side), (0, 0, 0, 0))
    square.paste(img, ((side - img.width) // 2, (side - img.height) // 2))
    square.resize((size, size), Image.LANCZOS).save(dst, optimize=True)
    kb = os.path.getsize(dst) / 1024
    print(f"{size}x{size} px, {kb:.1f} KB, under 128 KB: {kb < 128}")

slack_ready(sys.argv[1], sys.argv[2])

Run it with python slack_ready.py ai-output.png emoji.png. On a 1024 x 1024 test image of a green face on white, it printed 128x128 px, 7.7 KB, under 128 KB: True. One catch we hit: white areas inside the drawing, like the whites of the eyes, turn transparent too. Ask the AI for a background color that is not in the drawing, or use the emoji maker, which clears only the color touching the edges.

Why an AI emoji does not upload or looks wrong

• The GIF is too big. A still image of any size uploads, but an animated GIF must be under 128 KB after Slack shrinks it to 128 x 128. Fewer frames and colors help; see Slack GIF size limit.

• It has a white box around it. The AI drew a white background. Remove it, or it shows as a white square on dark mode.

• It is not square. Slack's help says square images work best. Pad a wide picture to a square so the subject stays large.

• It uploaded but does not show. See Slack custom emoji not showing.

• You cannot see Add Emoji. Your workspace admin may have limited who can add emoji; Slack's help says to ask an owner or admin.

FAQ

Is there an AI emoji generator built into Slack?

We found none. Slack's help on custom emoji covers uploading an image only, as of 28 September. Make the image with an AI tool, then upload it with Add Emoji.

What size should an AI emoji be for Slack?

Any size works for a still image; Slack stores it at 128 x 128. Crop it square with a transparent background. Animated GIFs must be under 128 KB after that resize.

Can I make an animated AI emoji for Slack?

Yes. Slack's help says GIFs can have up to 50 frames, though our September 30 test kept all 150 frames of a GIF. Generate a still image, then add an effect such as party or intensifies in the Slack emoji maker, which keeps the file under 128 KB.

Can I use ChatGPT to make a Slack emoji?

Yes. Ask for one subject, a thick outline and a plain background in a square image, then resize it and clear the background before you upload it.