Is Tumblrs even a word? Ah well, it is now.
Tumblr is a microblogging site used primarily to post pictures. It makes it very easy to add photos to your own stream, and its a great way to build a set of pictures to use in your game.
You can also easily check out other tumblrs, just by clicking on the link at the bottom-right of a picture that indicates the account it was linked from.
Here are several really interesting ones. I particularly like using Norse/Celtic themes in my games, so these are inclined in that direction.
The Stolen Lands
Fuck Yeah Vikings and Celts
The Deer and the Oak
The Iron Canyons
Swords and Arrows
As always, happy gaming!