I actually became a supporter myself recently. I haven't unlocked much but so far they've been just single images with a text and textless version, sadly. Some of them come with .psd file though.
You can buy rewards from previous months but it's a bit more expensive, 20% to be precise.
Kinda sucks because I really like his art, but it's just way too pricey. Especially since he posts pretty irregularly.