If you are asking about HOW to put it into a paper, then treat is just as you would treat it as a Figure.
In other words, in the manuscript you'd put
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Insert Figure X about here
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into the text, and then have the photo on a separate page at the end with the heading "Figure X" and the appropriate title.
(unless of course your instructor wants you to insert it directly into the text, in which case you'd still treat it just like a figure.
This advice is based on the APA style guide (i.e. the Publication Manual of the American Psychological Association - section 3.77 and 3.82). My copy is the 4th edition. There may be a newer edition out, but I have no reason to believe that it would have changed.
If you are asking about the legal way to cite it, I'm sure you'll be fine as long as you cite a source for where it came from - after all, it is jsut a paper for class, not something you are publishing for profit or public dissemination.