Would anyone like it?

I know it could be hard on some pc's or internet but thats why it should be default off.

I don't see why not... yet

The server only needs to downscale the gif and use it as thumbnail I don't think that its hard to make that happen

i would probably disable it in the settings since it takes me long enough to load that or any other form of animation (it'd make loading any searches much more longer for me) but i could see it being more useful than the old blank "animation" thumbnail for people who'd like a more visually descriptive way of knowing what said animation is about than just the tags.

biggest downside i could see coming from this idea would be longer load times when doing a search or something but that shouldn't be too mch of a problem for people so long as it's kept optional and could be toggled in the settings.