eAdventure Asset Library
A team from the e-UCM research laboratory has been long developping the open-source eAdventure platform. The eAdventure editor allows anyone with basic computer skills (no programming required!) to quickly develop and publish an adventure game. You might also be interested in checking out their examples.
Last year (2012) I had the chance to participate in the SEGAN's Summer School, hosted by the University of Zaragoza, and to give eAdventure a spin. To give us a chance to quicly test the editor without worrying too much about the graphical aspects, the eAdventure team created and provided us with a big asset archive which I will share here. It is licensed under Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
Since the archive structure is in spanish, this will hopefully not only help you prototype some serious game using eAdventure, or any other tool, but also review your basic spanish vocabulary!