This itch.io page looks awesome! You gotta give me some tips on how to do this!
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A lot of it is just trial & error. I used the main colors of the game for a lot of the main page elements, and then for the banner and background images, I drew specific artworks just for them. It took a lot of tweaking and re-exporting, especially for the looping background, but eventually I got it somewhere nice. Also, the rounded pixel corners at the top near the banner is actually part of the banner image itself.
As for the animated gifs: itch lets you upload screenshots while you're uploading a game, and it allows .gif files. In fact, I believe you can even do gif's for the banner and background, though I haven't tested that yet.
Sorry I'm a bit late to this. I had college start right as the submission period ended for the jam, so I've been a bit preoccupied as of late.