I'm doing some testing to see what the best format / upload site is.

This webp file was uploaded to an external site (not many support animated webp files, but ~~ImgBB is one I found that does).
I've used the full URL (rather than the direct hot-link), because using the hot-link often results in Lemmy pulling the file in and re-compressing it.~~
(edit: imgbb deleted the original post, have re-uploaded to catbox.moe instead. Whether lemmy lets a file stay on catbox, or copies it in and re-compresses it though, is difficult to determine)
An alternative is to make a text post, and hot-link the image as an inline link in the body. This has decent support on mobile clients, but it looks more like you've made a mistake in your upload for web clients.

Lossy .webp file, 1.16MB, uploaded to ImgBB

Desktop

web:
Thumbnail? yes, Loads in post? no, Loop? yes
alexandrite:
Thumbnail? yes (but cuts image), Loads in post? no, Loop? yes
photon:
Thumbnail? no, Loads in post? yes (but cuts image), Loop? yes
tesseract:
Thumbnail? no, Loads in post? yes, Loop? yes
mlmym:
Thumbnail? static image, Loads in post? no, Loop? yes (on click-through)

Mobile (thumbnail refers to Link View)

sync:
Thumbnail? yes, Loads in post? yes, Loop? yes
voyager:
Thumbnail? no (broken image), Loads in post? yes (but cuts image), Loop? yes
connect:
Thumbnail? no, Loads in post? no, Loop? yes (on click-through)
liftoff:
Thumbnail? yes, Loads in post? no (still thumbnail size), Loop? yes
jerboa:
Thumbnail? yes, Loads in post? no (still thumbnail size), Loop? yes
thunder:
Thumbnail? yes, Loads in post? yes, Loop? yes

I don't have access to every front-end and mobile app available. Please share your experiences with whatever it is you use. Thanks.