A gif is an image, so theoretically, uploading one should work the same as a large picture. Just right-click/save the original, download it to your hard drive, then re-upload to your journal -- making sure you've still got the "gif" extension attached.

If that's not working for you, PM me with the exact steps you're taking and we'll figure it out. :D

ETA: Nevermind. If your upload doesn't work with alexisjane's awesome instructions, it will never work. :D