First bookmark this tab https://worldlanguages.github.io/animatedThumbnailsBookmarklet/ then go to your scratch project you wanna have an animated thumbnail. Then click on the bookmark while your inside the project. Then it should show something that says: select image. Select the gif there. NOTE: BEFORE YOU DO THAT MAKE YOUR GIF AND DOWNLOAD IT create you gif here: gifmaker.me Also if you look inside you'll see animating code. You can use that to make animations. Just change the costumes to what you want it to be. If you want me to do more tutorials just ask!
This is called Full because some animated thumbnail tutorials don't teach you how to create the gif. Thanks to google for the animated thumbnail. Thanks to @FUNUT and @-MasterZ- for figuring out how to a animating thumbnail Thanks to @JCoder_16 for making the animating look a twincy bit nicer. When i say that i mean that he told me to center all the words making it look nicer