AI nail art preview from a hand photo: a mask_url edit on one finger

To preview nail art on a hand photo, send the photo and a design swatch as references to openai/gpt-image-2.5, add a mask_url that covers only the nails, and prompt for the colour and finish. The mask tells the model where to edit; check the skin around each nail before you show the result.

Treat the output as a preview. A render cannot promise how a gel finish or a nail shape will look on a real hand.

The mask rules that bite

Sume lists mask_url as an optional public HTTPS URL for ChatGPT Image 2.5 edits. OpenAI's image guide adds that, with several input images, the mask applies to the first image only, that the mask needs an alpha channel, and that it must match the target image's format and dimensions and stay under 50MB. So put the hand photo first in input_references and the swatch second.

{
  "model": "openai/gpt-image-2.5",
  "prompt": "Paint the nails in the masked area with the glossy burgundy chrome finish from the second reference. Keep the skin, rings and background exactly as in the first image.",
  "input_references": [
    {"type": "image_url", "image_url": {"url": "https://example.com/hand.png"}},
    {"type": "image_url", "image_url": {"url": "https://example.com/swatch.png"}}
  ],
  "mask_url": "https://example.com/hand-nails-mask.png",
  "aspect_ratio": "auto"
}

Mask checklist (read 2026-10-04)

Check the mask in code first

A mask that is a different size from the photo is the quickest way to get a wasted job. This check runs offline on two files you pass in.

import sys
from PIL import Image

photo = Image.open(sys.argv[1])
mask = Image.open(sys.argv[2])
print("photo", photo.size, photo.format)
print("mask ", mask.size, mask.format, mask.mode)
assert photo.size == mask.size, "mask must match the photo's dimensions"
assert "A" in mask.getbands(), "mask needs an alpha channel"
print("ok")

Edge review

Zoom to 200 percent on each fingertip. Look for colour spilling onto the skin, a cuticle line that moved, and a ring or knuckle that changed. If one nail is wrong, fix it with a second, smaller mask on that nail instead of redoing the whole hand.