I would say that if you want the postcard to hold the interest of your TARGET customers you really should use a more traditional "senior portrait" on the card. With the current picture one has to read the postcard to get to the point, and if any persons that may have been interested toss it without reading...
Look at it this way, if parents are shopping for a CLASS RING for their soon to be grad and they get a postcard with a huge picture of a CLASS RING it will catch their attention. If they get a postcard that shows generic jewelery they may toss it without reading the print, and never realize that the store has class rings on sale.
Also You have to be logical in order to compete with the pro companies, here is the way it is in our area. EVERY SINGLE incoming 12th grader has pictures taken by the "pro company" and is not charged a dime unless they choose to make a purchase(if not just call it yearbook pictures). The company makes its money on SALES of the prints not sitting fees, so as a parent "why would I take my child to you?"
Maybe if there was no obligation or cost along with that free print, but the postcard does not mention that...
I would also flip flop the 8x10 and 5x7 offers, give a free 5x7 with postcard and a free 8x10 for "refer a friend"(as referral must make purchase).