My girlfriend was just uploading GIF animation to FA and we were fighting a lot with thumbnail generation. 1) If she uploaded only GIF file, thumbnail generator used only last frame (transparency and difference from previous frame, default in nearly every software to save space) instead of proper image. I'm not sure which software FA uses, but for imagemagick there's option -coalesce which should solve this issue. 2) Worse thing is, if she uploaded GIF and separated JPEG thumbnail file, FA preferred generated thumbnail over uploaded one. We solved it by making animation in imagemagick with option -dispose none which makes working thumbnail, but pretty big files. If I try to search for "gif" on FA, it actually shows few of those broken thumbnails on art of various users, so it surely happened to more users.