Google has added the option to restrict search results to images that are licensed using Creative Commons.  Although the feature is not represented in the advanced search options, it’s possible by using the search included here:

FIND CREATIVE COMMONS IMAGES

It’s possible to tweak these searches by simply coping the URL, for example replace the word “Landscape” with any word or phrase you want to search:

1. Reuse
   http://images.google.com/images?q=landscape&as_rights=(cc_publicdomain|cc_attribute|cc_sharealike|cc_noncommercial|cc_nonderived)
   
2. Commercial reuse
   http://images.google.com/images?q=landscape&as_rights=(cc_publicdomain|cc_attribute|cc_sharealike|cc_nonderived).-(cc_noncommercial)
   
3. Reuse with modification
   http://images.google.com/images?q=landscape&as_rights=(cc_publicdomain|cc_attribute|cc_sharealike|cc_noncommercial).-(cc_nonderived)
4. Commercial reuse with modification
   http://images.google.com/images?q=landscape&as_rights=(cc_publicdomain|cc_attribute|cc_sharealike).-(cc_noncommercial|cc_nonderived)