This research can assign a time frame for the image and ultimately help you identify the individuals in the picture. You can find the photographer's imprint in a number of places, depending on the type of image. It's commonly found on the bottom edge of the photograph or on the back. Some photographers embossed the lower corner of an image with their name. Imprints usually include the photographer's surname and city where he or she worked.
Foreign photographs add another wrinkle, of course.
When researching foreign photographers you may need a language dictionary to help you decipher any printing or writing on the image. Finding the locality in an atlas or gazetteer can uncover the country in which the photographer worked. Just keep in mind that many place names changed or disappeared during political upheavals. You can attempt to research the photographer by using as much of the name as appeared on the image. You can also use city directories and census records if they are available for the location you're researching.