I would like to know. It seems like sometimes it works with some sites only on some Lemmy apps/website.

I’ve only had success with posting links to animated WebP files, which preview and display correctly in the default web UI. It doesn’t seem to work perfectly across the variety of apps and their handling of images though.

I think @NotAGuyInAHat mentioned that they use EZgif to do it. I generate them locally with FFmpeg, though that’s a little technical.