If you want animated thumbnails instead of static images all you need to do it upload an mp4 file to the cover area of your project, same as you would an image.
The mp4 file will need to be h.264 encoded. We do not optimise these files so recommend keeping the file sizes as small as possible as it WILL slow down the load time of the website.
As the video will be muted its best to remove audio to help reduce file size but then how you additionally try to keep the file small is up to you, you can reduce overall size or quality or length of time. No more than 2 seconds is really needed and you can probably get away with no larger than 1280 pixels wide, but really you want to be aiming to keep it under 2MB if achievable.
Also note how many you have on the page will effect load time. If theres a few ok but if theres 30 3MB files thats 90MB page download when the average website is under 4MB.