A GIF is unique among image files in that you can use it to make short animations made up of multiple still images. However, this can also drastically increase the file size, especially if the image is large or of high quality. Since tumblr has a 10MB upload limit as of publication, you might need to make larger GIFs a little smaller before uploading them, using a free image resizer like Picasion, Resize Pic or Ezgif.
