Create your own bumper banner with nothing more than paper and flour!
1 part flour mixed into 3-4 parts of boiling water makes an excellent glue suitable for bonding paper to metal or painted surface. It forms a strong bond but can be removed by soaking in hot water and rubbing.
Several strips of paper can be cut lengthwise into 3-4 inch wide strips to be pasted end-to-end to form a long banner (overlap them a little to reduce chance of edges lifting up).
You can use a laser or inkjet printer to print your message and graphics, or you can just draw by hand with markers or paint brush.
Regular printer paper works fine but photo paper (i.e. 32 pound weight) is a little hardier.
Of course, a paper banner will start to look shabby after a few rains, but it's a simple matter to make a new banner, perhaps with a fresh, timely new message. You can just layer the new banner on top of old.
Too much hassle? Consider buying a large removable paint-safe sticker instead. It only costs $11! See "Buy a sticker" for more info.