Most of the absortive panels are made using Corning OC703 insulation panelling, which can be purchased from industrial insulation suppliers (not your typical Home Depot item). Corner bass traps would also be a great addition and initially a good place to start. There's DIY instructions for building them online. The only problem there is the overall volume those traps need to be in order to be effective increases with the size of the room and with the range of bass you wish to address (lower = larger)...kind of a broad generalization, but also a caution that it is like betting on the horses to go out and buy or build some of this stuff and place it randomly hoping it will have the desired effect. If you are technically inclined, I'd do a whole bunch of research and try to get a grasp of just what you need to do for your specific room and problems. Then go the DIY route to make things look the way you'd like them to and possibly save some money. That or consult with an expert like Rives. As far as covering acoustic foam with posterboard art - you will be blocking out the effect of the foam as the board is not acoustically transparent. There's a fabric company in Maine, I believe, who makes acoustically transparent fabric. Not cheap, of course. You could just paint the foam itself - not sure what effect that may have, and, at least to me, it does not sound very attractive. Kal gave you some good resources for economical finished products.