Odaibako is a Japanese site where people can ask questions or make commissions, usually for free and with no time constraints, so artists tend to pick ideas that align with their work and draw stuff when they feel like it. If you search for posts with お題箱 or odaibako in the commentary you'll find posts that were commissioned there and some linking to the requests.
Commission says it's for images that were drawn for a paid commission, so I'm not sure about creating and populating this or simply tagging the posts with commission.