In essence, a primary source is a document written at the time to which it refers – a census return, a diary, a letter, a tax-form; while a secondary source is an interpretation of history – a newspaper, a history book, another biography. A secondary source may be contemporary with the event it describes or it may be much later, and there are clearly gradations of value in secondary sources.
from Writing Biography & Autobiography by Brian D Osborne
I think an image printed for distribution at a period in history would certainly be reliable, then--but is the caption? Here's an example from an old postcard I just acquired:
Clearly labeled "On the Bear Creek Canon Road"-- but there's reason to question. Most of the original Bear Creek Canyon wagon road, to the best of my knowledge, was low in the valley. A view such as this one should not have been possible. In fact, the road was so low it was flooded in 1933 and rebuilt higher on the canyon wall.
What do you think?