I'm not aware of any distinct collections of photographs of German POWs/internees. Are you looking for photos of particular individuals or just general photographs of activities/operations?
There are a lot of digitized photographs from World War I and World War II available on the The National Archives Catalog. If you're looking for more general photographs, you may want to start with a keyword search in the catalog (e,g. "German Prisoners") and then use the filters on the side menu to filter the results by Type of Material (Photographs and Other Graphic Material).
If you're searching for a particular POW, the personnel records of World War II German prisoners of war were returned to Germany. For access to those records, you would need to write to the Deutsche Dienstelle (WASt), Postfach 51 06 57, D-13400 Berlin, Germany. The website is http://www.dd-wast.de/.
Where I am at the Textual Reference Archives II Branch in College Park, MD, we have a series of Records Relating to German Civilian Internees, 1941-1946 (Record Group 389, Entry A1 466-G). The files are arranged by surname and occasionally include a photograph of the individual. There is a description on the National Archives Catalog at: https://catalog.archives.gov/id/876184, but the records are not available online.
If a POW was tried for war crimes there may be photographs scattered among the textual records in the Records of the United States Army, Europe (Record Group 549) or the Records of the Office of the Judge Advocate General (Record Group 153).
For World War I, we also have some records related to Prisoners and Alien Enemies, including a series of "201 files," https://catalog.archives.gov/id/7933768, but they may not contain photographs.
We hope this helps. If you still have questions, you could also contact NARA's Still Picture Branch (RDSS) by e-mail at: email@example.com.