Like Flickr, Picasa allows users to assign a Creative Commons license to their photos. Until recently, you couldn’t search for CC licensed photos, but now that has been added!
When you do a seach, just click Show options on the left side. Then click Creative Commons in the options and you can choose which type of license to filter by.
(To apply a CC license to photos you upload, click the edit button under Photo reuse to the right of your photo. To set CC as your default, to go your Settings and click the Privacy and Permissions tab.)
Thanks go Picasa and Google for adding this!
Picasa adds CC search