WebM works with image-rs (as long as you don't exceed your home instance's upload size limits).

I can't guarantee that every client can view it properly

that's on the client's author

but it'll show up in the Lemmy Web UI.

$ wget https://gnuplotting.org/figs/bessel.gif
$ ffmpeg -i bessel.gif bessel.webm

Try downloading that and then uploading it in a comment to to your own instance; it should work unless pawb.social limits the size.