I've noticed one of the major problems with gifs on Tumblr is them being frozen. The reason for them freezing is because the file size is 1MB or larger. This can get very tedious when you're trying to post a graphic, sidebar, or gif set. But have no fear, I found the solution. Here is a website I found called GifReducer that does exactly that. It doesn't effect how the gif looks, it just makes the file size smaller. I'm sure that there's other ways to do this but to me this seemed like the quickest and easiest way. Hopefully this works for you!
Heading off for a while. I need to rest soon. See ya guys later