You can't just upload an animated gif, unfortunately... That would be too easy for imgflip ;) You have to either upload a video, a URL to a video, or the individual images. If you have an animated gif already, then you need to split it... ezgif.com is an easy online splitting tool. You download the zip of split images, extract it, the upload those images to imgflip. Lastly adjust the delay timer between images, usually it might be 0.10 seconds but adjust as needed.