I teach third grade and my classroom is totally decked out with monkeys. I don't stamp my welcome postcards, but I do stamp my welcome bags that are sitting on the kids' desks on the first day. Here's the bag I've done the last few years...
Monkey Bag. I fill the bags with monkey pencils, monkey things from Oriental Trading, and a healthy snack for the first day of school.
For me, I create bags instead of postcards because I think they are super fast and I don't feel so bad when they end up in the trash. I used to stamp all sorts of things for my students (thank you's, bookmarks), only to see them on the floor, shoved into desks, in the trash can 3 months later when we do a desk clean out, etc. I don't stamp for my students anymore!
Do you have a classroom theme that you could incorporate into your stamped item?