You can use animated GIFs for your Mastodon profile's avatar and header image. However, these may not animate automatically for some people depending on their settings or app. If they don't animate automatically, they can usually be activated by clicking on them or hovering a mouse pointer over them.
⚠️ DO NOT use any kind of rapidly flashing animations! These can be extremely dangerous for people with certain neurological conditions. More info at https://fedi.tips/dont-use-flashing-images-or-emoji