glad my 8am ass can still write valid code. i did notice on your page that on a 1080p monitor there are actually too few buttons to make the marquee run smoothly. not much you can do about that other than to add more buttons. or i guess maybe set an explicit width for the parent container (in this case 621px or less since the buttons total to 1243px in width)
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 :)
obrigada!! eu gosto tua site :)