Look at the embossing on the front or back of the photograph mount board. This can provide the name of the photographer and the location. You can do a search on line using google or another search engine with he name of the photographer OR search for photographers in a location by date. You can search with the library of congress or a local genealogical society that can assist with photographers for that location for a certain time frame. Or try a University library with special collections section that might have a finding aid for a photographer. You can look for old city directories that list businesses in a location and see if you can find them. Smaller mount boards tend to be from about 1865 to 1870s then cabinet portrait ( Cabinet size mount boards ca 1880s-90s). Clothing - mens fashion for suits didn't change much. Sometimes had the same suit or a few suits for many years. Women changed their clothing and hair much more frequently. Newer photos that are not mounted will be harder ( after the turn of the century) when Kodak box cameras came out and people did their own and sent them back to be developed.
Then it comes to clothing, shoes, hairstyles, ages of people. What is in the background? Any events? Any landmarks that can be identified? You can send an email to a newspaper for letters to the editor. Maybe someone will reply via snail mail ( regular post) or via email or maybe you can look on line. Check a local library holdings on line for a photographer. Someone may have written a book about local photographers in an area.
abyznews.com - list of media outlets around the globe. www.libdex.com - access to libraries all over the place and you can search their holdings too.
Now censuses - get family names and dates +/- five years. generally people got married back then between 18 and age 25. Usually 20 or 21. Children usually born a year or two after. Get names of households. Then age of children and spouses. Then re-look at the photographs and see if you can narrow things down in group photos. There are usually clues somewhere in the photo. Even the place or background/backdrop used in the photo.
Probated wills. City directories. Phones may not have existed before a certain time. But localities did have a farmers directory or a local business directory that you can look at. Local Libraries. Historical Societies. Genealogical Societies. Universities and State Archives. City records. Including Land records Newspapers on microfilm or microfiche. Look for an advertisement for the photographer.
Finding out who they are might be a problem. But if you can narrow it down, maybe it will work it self through other clues. Remember.. Some photographers did travel or photograph in different places. That is why you should check city directories and historical societies. Sometimes a building may have held a photographic business throughout its existence. One photographer that was local to me had been in business for over ten years in his building. But going through historic records, that same address had a photographer in it since the 1890s. A photographer started out, had a son, ran it together until the 1940's until the father passed away. Son kept operating until retired and sold it to someone else, change of name and ownership.