I have collected many items, but I haven't really bothered with the rare paintings yet, still haven't found impact, creation of the guide, waldo, and sparky (Currently unobtainable.) I talked to the developers, and they may buff the spawn rates of those paintings in the future. I have beaten ML on expert once, but I'm about to go into another expert mode playthrough! I've obtained the nymph banner, the sea snail banner, undead miner banner, corrupt penguin banner (very easy to obtain, just throw vile powder on penguins spawned from a penguin statue. I haven't had troubles obtaining any worm type banners, but that's just me. I've mainly focused on my PHM character, where I have over 200 platinum, thousands of gems, over 12 stacks of heart crystals, and just sheer numbers of items, I just like collecting reagents and whatnot, I haven't really focused on rare items in a while, but my next expert mode playthrough will return me to those roots.