This often neglected type of research is valuable to not only find a photo of an ancestor in a library or archive, it also helps to provide a historical context for their lives.
Anyway, take a look at the brand spankin' new Photos! resources page. It is full of several informational articles, many resource links, as well as links to online image and photo collections that contain millions of photographic images - all about researching photos to find articles about our ancestors and their surroundings.
So check it out!