go to this link: https://is.gd/customthumbnail there is a blue link on this page. drag it onto your bookmarks bar. the bookmark should be called: "Animated thumbnail set" when it is bookmarked. go to a shared project, click see inside, and click on the new bookmark. make sure you have a gif file downloaded that is 900kb or less, and 500x500 pixels or smaller. click on the first blue set of words and choose the gif from your files. wait until it shows the gif in a little box at the top. it will notify you if the gif does not work. go to your scratcher page or my stuff page and look at the project. if it isn't animated, then try again. Do not go to the project page after setting the gif, go straight to the scratcher page and then to the project, or right after sharing the project to put the instructions and credits. If you go to the project page directly, then the thumbnail will be set as what you left it as, instead of the gif.
works best on crome you can change the gif anytime using the same steps as above. if you don't like how i explained it, then search it up on youtube it explains it pretty good there too