As an artist that has work on Roll20's Marketplace, here's how the procedure went. First I found out about the Marketplace, and I emailed the staff, to which Nolan replied almost right away. He asked for some samples of previous work, to which I sent him a few maps. After ensuring quality control, and having me sign a contract, he informed me that the best way to do things was to create a pack, create a cover image, and then contact him. At which point, every time I have a map pack ready to go, focused on a particular theme (such as dungeon tiles, wilderness tiles, etc.) I shoot him an email with the pack description, set price and the cover image, which he then creates a space on the server and sends me a link. I then begin uploading images, which may take 1-2 hours depending on how large the images are, how many I have, if I'm doing something else simultaneously (like watching a movie, writing, or doing other artwork), or if there's connection issues. After everything is uploaded, I send Nolan another email, essentially saying "It's ready!" and he sets the pack to be live for download. The staff has zero input on when I create a set, what I include in a set (as long as it's appropriate), the theme to the set, etc. The frequency of such creation is completely random, as my first set was uploaded in February of 2013, the second in March of 2013, the third in June of 2013, the fourth in September of 2013, the fifth in February of 2014, and the most recent less than a week ago. Essentially it's whenever I have the time and desire to create something for Roll20, or when I have enough content from my own games that I feel will fill a pack that will sell.