The National Library of Scotland has launched an online collection of propaganda that was used during World War II. The collection is divided into “white propaganda” (they define as “mostly practical information intended for the Home Front”) and “black propaganda,” which targeted enemy morale.
From http://www.nls.uk/propaganda/index.html, you can read an overview of the collection and select white or black propaganda. Each link selected will give you a more general overview of the topic as well as a thumbnails to propaganda images — not posters, but rather booklets and postcards in the case of white propaganda and leaflets in the case of black propaganda. Clicking on the images brings larger images and detail pages about the item. The content of the item itself is not available online.
The collection is rather small but I found it interesting as most online collections of this type tend to focus on posters.