It seems that Britain is in the process of digitizing almost 2,000 propaganda posters and paintings from World War II. Already 363 have been uploaded to Wikimedia Commons, and the remainder will be added shortly. Here are a few examples from those already online.
There are many more images at the link, all available in very large sizes (suitable for wallpaper or screen-saver use, as well as printing). Highly recommended viewing for history buffs.