>>7337322/wg/ has a max filesize of 8MB IIRC. That's too small for genuinely large resolutions. Lossless 4k is usually far too large to post on /wg/, never mind anything larger. You'd have to JPEG compress it to the point where it'll look terrible despite the resolution. I have a 10,000x6,000 composite made by the curiosity rover, but it's 43MB. I don't think Windows can even use wallpapers that large.
