This picture is a compilation of all the artwork used during co-op games when a boss even is about to be started (The 1 minute confirmation/teleport timer you get before the skeleton king, for those of you who tried it in beta). As far as I know, this picture contains artworks of all the bosses of the game and is a pretty huge spoiler, click at your own risks.
Battle.net Profile Boss Icons
Same as above, the new player profiles have extra assets to display the progression of the player, including a nice portrait of each end boss with different color for the eyes depending on the max difficulty you reached.
For those of you who want to hurt their eyes, most of the pictures for the waypoints are also available in the game files and could give you an idea of what the zones of the game will be. These pictures most likely include all the Acts of the game, once again, spoiler.
Monsters and NPC Portraits
The game files also have character portraits, those are already known to the public and were posted on forums a while back but I'm re-adding them for completion sake.