Level-Design.org started up a new database for reference
"Reference Database is a free collection of game screenshots with assigned tags that describe the images’ content. This database answers the question "How it was made in games?” so if you look for the rocks just type it in the search box and the result will be a list of game screenshots with rocks. It will help you plan and execute design ideas or the look of your graphics by setting up a benchmark – the reference from different game titles.”

http://www.youtube.com/watch?feature=player_embedded&v=Cb7QJwQ58T0