Picasa Web Albums now allows you to filter search results: Just click on "show options" to enable the tool belt with advanced filters, then choose an aspect ratio (landscape, portrait, panorama), select a size (small, medium, large or extra large), filter videos and view Creative Commons licensed files. Like Flickr advanced search, and Yahoo! Search, you can use Picasa to find Creative Commons-licensed images that can be reused or modified.

Source:→ Google OS