Slack custom emoji under 128 KB: shrink a Sume image to fit

Slack's help page says custom emoji images should be square and under 128 KB, and it accepts JPG, PNG or GIF; transparent backgrounds work best. The page gives no pixel dimensions, so pick a small square, such as 128 x 128, and let the 128 KB byte limit decide whether it passes. A 1:1 request on Sume plus a Pillow resize does it.

What the page states

Animated GIFs can have up to 50 frames. For a still, the two checks are shape and weight.

Getting a transparent emoji from Sume

Use ChatGPT Image 2.5 with background: "transparent" if you want a cut-out; the Image API docs list that parameter for that model, and Image 1.0 with transparency: true is the other documented route. Keep the prompt to one bold object with no text. Fine detail turns to mush at 128 pixels, and Slack shows emoji far smaller than that in messages.

Scale, quantize, check

A PNG quantized to a smaller palette is much lighter than a 32-bit one. The script resizes to 128 x 128, then tries palettes of 256, 128, 64 and 32 colors until the file is under 128 KB.

import os, io, requests
from PIL import Image
r = requests.post(
    "https://api.sume.com/v1/images",
    headers={"Authorization": f"Bearer {os.environ['SUME_API_KEY']}"},
    json={"model": "openai/gpt-image-2.5", "aspect_ratio": "1:1", "prompt": "one cheerful cartoon star, flat colors, thick outline, no text", "background": "transparent", "output_format": "png"},
    timeout=90,
)
r.raise_for_status()
if r.status_code != 200:
    raise SystemExit("202: read the finished job from /v1/jobs/{id}/result")
img = Image.open(io.BytesIO(requests.get(r.json()["data"][0]["url"], timeout=60).content))
art = img.convert("RGBA").resize((128, 128), Image.LANCZOS)
for colors in (256, 128, 64, 32):
    art.quantize(colors=colors, method=Image.Quantize.FASTOCTREE).save("emoji.png", optimize=True)
    if os.path.getsize("emoji.png") < 128 * 1024:
        break
print(os.path.getsize("emoji.png"), "bytes with", colors, "colors")

What makes a good emoji image

Emoji are viewed at a tiny size next to text, so the image has to survive reduction. One subject, a thick outline, flat color and a transparent background hold up best, which matches Slack's note that transparent backgrounds work best. Avoid gradients and fine texture; they raise the file size and add nothing at that scale. Slack gives no pixel size on the page, so 128 x 128 is a choice, not a rule; if a file is under the limit at that size you can try a larger one, but there is little benefit. Name emoji by what they show. Because the page names the 128 KB budget and the types JPG, PNG and GIF, avoid WebP, which Slack does not list. If a generation has a messy edge or leftover background pixels, regenerate it with a plainer prompt instead of cleaning it by hand.

Cost of a set

Emoji sets are many small images, so the cost is n separate requests, each billed by usage.cost. The catalog also lets one call ask for several images with n, up to 10, though each model has its own lower ceiling in the catalog. Read the model's n range before relying on it. For a longer walk-through of the workflow, see how to make a Slack emoji with AI.

The quantize loop prints how many colors it needed. If even 32 colors do not fit, the image is too detailed, and simplifying the prompt is the better fix.