Steam Library Capsule 600x900: below the image pixel floor

Steam's Library Capsule is 600 x 900 PNG, but 600x900 is 540,000 pixels and the Sume image docs set a floor of 655,360, so it is not a valid custom size. Generate 800x1200 (same 2:3 ratio, 960,000 pixels) and downscale to 600x900.

Steam sizes are from the Steamworks library-assets page; the limits are from Image models.

Why does 600x900 fail?

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. It also fails the multiple-of-16 rule (600 / 16 = 37.5, 900 / 16 = 56.25), so there are two reasons. The table below lists the candidates.

Is there a Timeline route for exact 600x900?

Perhaps. Timeline output width and height are even integers 256 to 2160, so 600x900 is allowed there, and a video[] source may be a still, which the docs call a static hold. The output is an MP4, so you would then pull a frame with video frames, whose png format is the lossless choice and which keeps the source size when max_edge is omitted. That is a longer path than resizing a PNG yourself, so use it only if you want the whole thing in one API pipeline. Check the output dimensions.

What about the Library Header at 920x430?

It has the same problem: 920 x 430 is 395,600 pixels, below the floor, and neither edge is a multiple of 16. Pick a larger size with the same shape, or downscale from one; see Steam capsule sizes for the store-page assets.

What should I do?

Generate 800x1200, check that the cover art and logo survive the downscale, and export to 600x900 PNG in your own tooling. Steam notes that the half-size 300x450 capsule is generated automatically, so you do not need to make it.