GIFs have been obsolete for a decade now but they've never really gone away. Even as bandwidth and browsers are more than capable of handling high quality images, GIFs have a few qualities that are really their own, transparency, small file size and so on.
More recently though, it's one thing that made them stand out the most, GIF is the only widely supported image format that handles animation.
As you've surely noticed by now, animated GIFs have moved beyond the forums that nurtured them for many years and have become "cool," even pieces of art.
And, as with any art form, it's not long until people want to capitalize
on it. Hence, GIF ads on Tumblr, where else. To be fair, using GIFs for ads is smart, for one they don't feel out of place on Tumblr while at the same time grabbing a lot more attention than a static image. You can check out some of them in the gallery below, but you should spot more in the Tumblr Radar.