Do I have to buy pictures from the official prom photographer?
You don't have to buy the official pictures that they take at the prom. In fact, if you're on budget you probably don't want to because its going to be anywhere from $30, maybe even $100, depending on your school, so stick with your own camera. If you really want to, if you want to do sort of a: 'I went to Merrill High School and everywhere I went it was Class of 2001', then do it, but if you don't have to, with Photoshop now its probably just as good on your own anyway.