Many Tumblrs seem to have problems when they try to add, display and posting an animated gif, even when the size is small enough it appears to show up as a regular jpeg pic. We found a couple of tricks that make sure your animated gif picture gets inserted and shows up correctly in your Tumblr.
I recommend you try the following method to put and display an animated gif picture in your Tumblr blog.
How To Add an Animated Gif to your Tumblr.
Publishing a Photo/Text Post and Inserting an Image
Tumblr seems to have problems when handling animated gif that are “bigger than expected”, I’m not exactly talking about the size in memory but the actual width and height in pixels. First of all, what you always must do is host the animated gif in an external hosting site, I personally recommend TinyPic. This site never has problems when trying to display animated gif images. Now, I suggest you use the Text Post Type instead of the photo one, I don’t know why but there are fewer issues when you put your animated gif this way. One in the post click on insert image and paste the animted gif url that TinyPic gave you, click on insert and publish.
That’s it, if you are still having trouble getting your animated gif displayed this way leave us a message with the problem description.