I think the ideal upload would:

  • Work properly amongst the widest range of clients (the basic website view, the various frontends, the Android apps and the iOS apps).
  • Display a thumbnail of a still or animated image in the thumbnail (the little image in a ‘List’ view).
  • Load directly in the post (when expanding a thumbnail, or when using a ‘Cards’ type of view).
  • Loop indefinitely (if that’s what you want)

Of the formats (GIFs, WEBPs, and MP4s, it’s currently GIFS that get closest to reaching this ideal).

Lemmy and its clients will likely improve in the future, but - for now - here’s some notes on the formats:

GIFs:
  • Can’t be uploaded directly to lemmy (they’ll be converted into an mp4)
  • External sites that’ll accept gifs include giphy.com and tenor.com
  • Sync will try to download media and render it for fullscreen viewing. Giphy doesn’t like that, and serves up a ‘content not available’ gif instead.
  • Lemmy will make an arbitrary decision whether to leave a file on an external site, or download it in order to cache it. If it does, it’ll convert it to a mp4, but then the basic web view doesn’t know what to do with it, and renders it as a static image.
WEBPs:
  • Can be uploaded to lemmy or to an external site (e.g. catbox.moe, or imgbb.com)
  • If uploaded to lemmy, lemmy will always re-transcode it, affecting the quality (sometimes drastically).
  • The same will happen if lemmy decides to copy in the external file.
  • Apps like Sync (on Android) and Mlem, memmy, and Lemmios (on iOS) can struggle with them, rendering them as blank or static.
MP4s:
  • Can’t be uploaded directly to lemmy
  • External sites that’ll accept mp4s include imgur.com
  • Very unlikely to provide a thumbnail
  • Minimal-to-zero support in iOS apps
  • Don’t loop by default when viewed from a website (can be changed by a right-click option though).

Thanks again to the people who helped with testing.

Of the frontends available - Photon or Tesseract (‘a heavily modded fork of photon’) are the best.
The others - MLMYM (‘old’ lemmy) and Alexandrite tend to require an extra click to see things.
The basic web view suffers when lemmy breaks things by trying to copy them in.

Of the Android apps - I’ve found the PWA phtn.app to be the best, and would also recommend Thunder (although it requires an extra click for MP4s) and Voyager (although it doesn’t handle GIFs broken by lemmy).
Sync struggles with WEBP files, and doesn’t play well with Giphy; Liftoff, Jerboa and Connect don’t play WEBP file fullsize (or at all) in the post. Eternity has generally poor support for anything animated, and Jerboa can’t natively handle MP4s.

Of the Apple apps - I don’t have iOS, so I can only go on what others (like ApathyTree, Hawke and itsathursday) have reported
Support for MP4 seems generally poor, but for GIFs and WEBPs, Liftoff, Voyager, and Avalon seem to be the best.
Lemmios and Mlem can’t play GIFs and open animated WEBP files as static images (as does Memmy)