my way might be a tad janky but basically you gotta have 2 instances of your images in a regular element (not [marquee]) and then use a CSS animation to scroll the element to 50% and instantly reset back to 0. this is the code (https://pastebin.com/V6qMYmFP), but be wary of it because i wrote it on maybe 4 hours of sleep.

Ahhh shucks that is so sweet, thank you so much! i'd be happy to explain, do you have an email address? then i can explain it over email, which is easier :)