Try your local Walgreens for cheap passport photos. But if you're really that cheap, take your own. As long as they meet the size requirements and only show your head, you'll be fine.
But why get a passport if you don't need one?
If you're doing a closed loop cruise (leaving from and returning to the same U.S. port and going to the Bahamas, Caribbean, Mexico, etc), you don't need a passport. You can get by with an original passport.
Talk to your agent and they can tell you what you need.