I also want to add some artwork to my game and just yesterday I reached the CoG staff with a similar question, here is the most relevant part of the answer that might help you.
“There is not a requirement for the internal art in games, but we do recommend that the files stay small, and generally JPEG images. Currently we do not support GIF files in games. You can include as much art as is reasonable in your game, but please be aware that too many images can cause a game to load slowly or crash”
The image sizes used are still the ones MeltingPenguins showed us on this post
I have made some polls about what people preferred to see regarding the art in the game. You can find that here.
