Here's a little program I made to easily create a boot animation for Marlin. I used it to make my printer play ~40 seconds of Bad Apple on boot, you can see it here: https://www.reddit.com/r/ender3/comments/13b0pjw/it_only_too...

For anyone digging through random forum threads 9 years in the future, trying to figure out how to do animated boot screens: Animated boot screens appear to be documented very little, so I intend to do a write-up of it on my site, https://askiiart.net eventually. I'll reply to this post with the link to that, expect it in the next few weeks.