So I uploaded a GIF to Tumblr, and the mini version is animated, but the regular version is not. Why?
We had a problem with people uploading massive animated GIFs that, even when resized, would be inconveniently huge (1+ MB) and would slow down Dashboard rendering even on broadband. (You can fit a lot of animation within the 10 MB photo-upload limit. Some people were using it to post short video clips.)
We didn’t want to disable animation in our images — there are too many legitimate uses. But we wanted to make sure that people weren’t getting these huge images in their Dashboards, either.
So there’s a new method: any PNG or GIF for which the resized output is larger than 500 KB is converted to a high-quality JPEG instead.