Steam Library Hero 3840x1240: over the 3:1 image limit

Steam's Library Hero is 3840 x 1240 PNG, and the Sume image API will not take that size as custom pixels: 3840 / 1240 is about 3.10, above the 3:1 maximum. Request 3840x1280 (exactly 3:1) and crop 40 px of height to reach 1240.

Steam's asset sizes are from the Steamworks library-assets page; Sume's limits are from Image models.

Which sizes pass the custom-size checks?

The docs say, for the ChatGPT Image 2.5 models, custom pixels need both edges to be multiples of 16, a maximum edge of 3840, an aspect ratio of at most 3:1, and 655,360 to 8,294,400 pixels. The table below puts 3840x1240 beside the nearest passing size.

Where does the Steam safe area sit?

Steam lists a safe area of 860 x 380 at the center of the hero. Keep anything important, such as a face, inside that box; Steam also overlays the library logo on the hero. Cropping 20 px from the top and bottom of a 3840x1280 image leaves the center, and the safe area, unmoved.

Do I need the half-size too?

Steam says the half-size 1920x620 is generated automatically from the hero. If you need to make your own, 1920x620 is 1,190,400 pixels, which is inside the pixel range, but 620 is not a multiple of 16, so it is not a valid custom size for the image API either. Resize from the full-size file instead.

What should I do?

Generate 3840x1280, crop to 3840x1240 in your own tooling, and export PNG, which the output_format field supports. If your source is a video, video frames can pull a frame with an optional long-edge clamp of 16 to 2160 and keeps the source size when omitted. See also Steam capsule sizes.