I have made art for 2 future Roll20 creator's adventure oneshot modules in exchange for a % of their sales until the end of time. Otherwise you pay someone straight up, find a stock art you like, or make your own art. If you get FilterForge and stock art, that's the secret many people use to make cheap and easy art. Or they do 3D art models, it's possible to buy premade models that you pose for example, but that's a more expensive way about going about it. I draw top-down tokens and use painted over stock for face-forward tokens. Honestly, it's not that hard to make a map. The vast majority of people use photoshopping techniques to do their maps, or programs that come preloaded with stock assets like Wonderdraft, Dungeon Forge, Other World Mapper, and the well known Campaign Cartographer 3+ and the many add-ons and additional programs from those creators specifically for map making without being able to draw. Or you can learn to draw! But you really don't have to be able to draw to make any of these things, just grab the program of your choice and give it a whirl! Battlemaps for basic oneshots are the sort of work you can do by spending a day working on it, they're not the 40+ hour adventure that a city map done to spec can be. o_o For the record, myself, I would license any of my tokens for reuse in a Roll20 marketplace adventure for a minor additional fee, same with my maps, and make arrangements for a % for custom art... depends if it's something I want to make or 'be part of', I definitely want DMs to be able to release their oneshots for sale here! It's worth it to ask if there's a market creator who has a package you think is perfect for the monsters in your written adventure, but always ask instead of assuming it's okay.