posts to tumblr are not animated

Hi, when I use the flixel app on my iphone 4gs and post directly to tumblr from the app, my gifs are not animated when shown on tumblr.  I believe tumblr or the flixel app might be resizing the images.  How do I get it to post and maintain the animation?  I've tried both using the built in post to Tumblr functionality of the app and also simply emailing the gif into tumblr to post that way.  Neither works.

Thanks!