Use The Commons search feature to limit your search results to the image collections of cultural institutions like museums and libraries; images use the licensed as having "no known copyright restrictions." Read more about the rights statement here. Make sure to select "The Commons" from the drop-down menu in the search box.
By using Google's Advanced Search feature, you can conduct a Google Images search that will limit your results to only those images that carry a label for reuse.
Types of reusable content includes images that are labeled with a Creative Commons license, images that are labeled public domain, and items labeled with a GNU Free Documentation license. Note that as these designations all differ--from the public domain to the varying degrees of restriction provided for through Creative Commons licensing--it is important to review the license attached to the image before you make use of it.
To limit your results, select "Only images labeled for reuse" unter the Usage Rights feature of the Advanced Search.
Depending on your needs, you may also decide to further customize your search. For example, you may choose to limit your results to only images that are labeled for "Commercial Reuse with Modification." Read more about the Usage Rights search options.
Wikimedia Commons is a media file repository making available public domain and freely-licensed educational media content (images, sound and video clips) to everyone, in their own language. Search by topic, location, type (images, sound, video) and more. There is also a link on how to cite when you reuse content outside Wikimedia.
An image search on theater will automatically return results for both theater & theatre, as well as a suggestive category search for the term below the title. A brief definition is included, with related image galleries linked to the page for additional search results. The various images provided are listed at the bottom, and when selected will provide a summary of the image and how to download.