Vermeer and Caravaggio possibly having used the camera obscura to create some of their famous paintings. Other painters are apparently well known to have photographed their subject or to have used someone else's photograph.
Pictured above is Paul Gauguin's "Mother and Daughter" and the Henry LeMasson photograph that served as its model. Other examples are compiled here and discussed in detail (with an occasionally acrimonious comment thread) here.
I don't think these discoveries or acknowledgemens lessen the quality or importance of the artists' work. To me, they are just using the technology of the time to ease and enhance production of the work.