How to add multiple pictures into video scene using Sony Vegas 10

I am creating a Christmas video for a school. In one of the scenes, the character is thinking of several students. In that part of the scene where the main character is thinking of and naming each of seven students, I want that student's picture to pop up in an "idea bubble" until all seven students are pictured above the main character's head. Is this do-able? Do I do it with masks or some other method? Help! I have to do this next week.