Hanging pictures..

Partner is enjoying painting (as mentioned before), and we have ended up with a boat load of pictures. There aren't worth spending money on framing them all, just the nice ones given out as presents, but we would like to put more on the walls. As these are as much practise as her expressing art, they are painted on 5mm mdf board for cheapness, in an approx 50x50cm size. Any thought on how to hang or display them? We can't screw into them physically, and I'm not sure if the likes of "No more nails" to bond string to them would work (were not sticking them directly to the wall!), I did wonder about a thin shelves in her office (art room) with enough room for them to sit on.

Does the hive mind have any idea's?