>>733
The problem here is that ffmpeg is made to work on videos, not on gifs that have things like
+ variable framerate (where multiple frames can be displayed at the same time)
+ frames that only update/replace part of the image
+ different palette per frame... which combined with the previous two allows you to actually have more than 256 colors in a single frame
ffmpeg will internally flatten all this out into a series of single resolution frames, then downscale it using some interpolation (so even if the input gif didn't use any of these tricks, it'll still have tens of thousands of colors), which then will have to be quantized, resulting in thumbnails that are huge and look crap...
There's this thing I wrote a couple of years ago, this understands the gif structure and only does nearest interpolation (so it won't blow up the number of colors, even if it looks a bit worse), but needs some testing, and of course to be integrated into jschan.
https://gitgud.io/some_random_guy/gifthumb
Also at the moment it fails on 2 gifs on this site. From the error message I have a feeling that they're actually corrupt and it's just giflib being a bit more picky then other libraries/programs, but I didn't look into it in detail yet.