This is explained in more detail in a piece published to BuzzFeed News on April 27th, 2017. If a Twitter user has a GIF profile picture, they likely uploaded the profile picture when the loophole was still active and haven't changed it since. Shortly after the loophole was discovered it was patched by Twitter devs, however, those who had already uploaded working GIF profile pictures found they still worked. BuzzFeed published an article describing how to do this on June 20th, 2012. Around June of 2012, a loophole on Twitter was found that allowed users to upload a working GIF as a profile picture by cropping it to a specific size.
