Yes, I've made them. I'm thinking about painting more of them and print the best ones as numbered and signed posters (offset). Offset printing isn't considered "genuine art quality" like a lithography and is therefore a lot more affordable way of reproducing images, and hence much more affordable for both buyer and seller. An offset print can be sold for about $20-$30 while a lithography is more like $100 and more depending on who signed it.
All of these are made digitally with PS or a great (and cheap, $25) software called Artrage. I also have a Wacom tablet which means it's almost like working with actual pen/brush/airbrush and paper or anything else you want to work on. The only real difference is that it's easier to correct mistakes.