Where did you get that photo for your book cover? Business lawyer Helen Sedgewick, author of the Self-Publisher Legal Handbook, runs a website-blog of articles on legalities concerning self publishing. In this article, read how you can legally use photos of identifiable people:
How to Use Images of Real People Without Violating Privacy and Publicity Rights
Note: Nowadays you must also be careful of commercial use of photos taken in parks and gardens with identifiable features. Check the location's website for rules on photography.