We put school name, address, and phone numbers(we have 2 - a general # and one you call when your child is absent). Last year we had our mascot but this year we put the PTO apple, I got it from discovery school clip art. Then we put "Thanks for supporting your PTO"
Ours just has the group name, mascot, school statement and a place for them to put their name. I know of some schools that use them as coupons. They get local businesses to allow them to put a deal on the back as an extra incentive for membership.