hi praireprincess.. I found a forum about this here: http://www.stevehoffman.tv/forums/archive/index.ph...
Honestly, I have no idea but have always wondered myself. I have used photos that were not "obviously" copyrighted and made sure to link and credit the photo without consequence but it still may not be ok to do.
I am interested in hearing a straight and clear answer to this as well and maybe some sources for photos to use. It seems as it should be okay especially if you are giving a good review but I guess you just never know.
Hope the link above helps!