it can depend on the software. matrix doesn't let you paste animating gifs. it just snaps the first frame and then converts it to jpg. even in the web app. most nostr web apps don't tinker with the files though. i used this, there is a canvas api in the web browser runtime that can take an image, decode it, then using the canvas api, it can scale it, warp it, and then encode it to jpg, gif, webp, or whatever.