I did this a few years ago with my son's school picture. We also wrote on the back side of the ornament the year and his age. Grandparents loved them. I copied the picture on to sort of heavy weight paper, you have to roll the picture to get it in the glass ball. Chop sticks come in handy for this process. You can use any kind of glass ball.
This family is ADORABLE! The Back to School Fairy comes every year the day before school and leave something for the kids so that they are excited for back to school. The blog also has TONS of great teacher gift ideas. I'd be okay if I have these kiddos in my class!