MassArt and its neighbor, the School of the Museum of Fine Arts, both have decent offerings on the continuing ed side, although they are weighted towards the summer. The quality of the profs will vary, howver, and to get at the big names teaching at either school, you may have to enroll. BU and Emerson have some limited continuing ed offerings. Unless you are looking to earn college credit, NESOP might me a more economical alternative, although MassArt may be almost as cheap, being a public College.
The Cambridge, Brookline and Boston Centers for Adult Education also generally have photo classes, but tend towards digital, and, again, the quality of instruction will vary widely.