GIFs Not Moving on Tumblrby Laurel Storm
The perfect animated GIF sometimes isn't. Oh, it may perfectly encapsulate your feelings or that one special moment of your favorite TV show, but then you upload it to Tumblr and it stubbornly refuses to actually animate. Deleting the post before anybody sees it is just the first step -- after that comes fixing the GIF so the problem doesn't happen again.
Cause and Effect
Tumblr enforces strict limits on both the pixel dimensions and the file size of images you upload. Animated GIFs in particular may not be wider than 500 pixels or larger than one megabyte. If an animated GIF exceeds the limit in file size, dimensions or both, Tumblr will automatically resize it once it's done uploading. This resizing is what turns the animated GIF into a static image, and it can't be avoided except by modifying your GIF so it doesn't exceed the limit.
Shrink the GIF
If your animated GIF is wider than 500 pixels, the first thing you do should always be to resize it, even if it's also more than 1MB in file size. This is because the pixel dimensions of an image are one of several factors that have an impact on its file size; so simply shrinking the GIF may be enough to also bring its size under the maximum limit. After you've resized the GIF, check its file size; if it's less than 1MB, try uploading it to Tumblr again to see if the issue is solved.
Reduce the File Size
If your animated GIF is still too large after you've changed its pixel dimensions, you need to reduce its file size. Doing this is going to take you some trial and error. You'll likely have to remove at least some of the frames or, if the animation is too long and complex to look good with removed frames, split the GIF into two. Reducing the number of colors used in the GIF's palette will also have an effect on its file size, although the GIF may no longer look as good with the smaller palette.
Upload It Elsewhere
If nothing you try can get the GIF below the limit without also ruining it, you do have one last option. You can simply upload the file to an image-hosting service such as Imgur, TinyPic or Photobucket and then insert it into your post from there without uploading it to Tumblr. This won't work if you want to use the GIF in a photo post, though -- images in photo posts are automatically uploaded to Tumblr even if you pull them in from a URL instead of uploading them from your computer.