By and large, the art resources to implement published board/card games on Roll20 are not going to be (legally) available. If you own the game and have a scanner, you could scan the pieces in yourself, for example, and run a game with friends, but spreading those images around the 'net would be a no-no. There might be certain specific games whose pieces or card images are in the public domain or are under some license that would allow you to share them, but that would be specific to that game, and I'm not aware of any such title. I, too, would love better deck manipulation capabilities. Even with the API, though, you can't use Roll20's actual decks system to achieve a deck-building game like Dominion or the Pathfinder Adventure Card Game. I've toyed around with building a deck system from scratch with manipulation capabilities for Betrayal at House on the Hill and Pathfinder, although I haven't completed either project. That said, some games do work decently well for porting to Roll20, even without API support. I believe Gauss has a Roll20 campaign set up for Munchkin, for example.
