An overview from the California State Library - http://www.library.ca.gov/goldrush/

An amazing collection of primary resources related to California - http://www.calisphere.universityofcalifornia.edu/

And just to show an example from the last database I mentioned, here is an example search resulting in pictures of old gold rush towns - http://www.oac.cdlib.org/view?docId=tf1489p0tv;developer=local;style=oac4;doc.view=items