Many search engines provide a specific image search. Not all images found in this way will be copyright free.
When using Google's image search click on 'Settings' and choose the appropriate option from the drop down list. This is usually either, "Labeled for noncommercial reuse" OR "Labeled for noncommercial reuse with modification"
Bing also has a license filter built into its image search that allows you to filter images by Creative Commons licence. This new feature incorporates Flickr and many other image sources. Click on 'Filter' then licence and choose from the drop down list.
When using the Yahoo image search you can also filter images by Creative Commons licence, Click on 'Any Licence' and choose from the drop down list.
Creative Commons Licenses.
You may copy, distribute, display and perform the work and make derivative works and remixes based on it only if you give the author or licensor the credits (attribution) in the manner specified by these.
You may copy, distribute, display, perform, and modify the work, as long as you distribute any modified work on the same terms. If you want to distribute modified works under other terms, you must get your permission first.
You may copy, distribute, display, perform, and modify and use the work for any purpose other than commercially unless you get permission first.
You may copy, distribute, display and perform only original copies of the work. If you want to modify the work, you must get permission first.