AT usually points to a location. I met him AT the elevator. I looked AT the picture. I am AT the store. I look AT the wall and see the ornamental patterns. It describes where something is located. ON usually means atop (on top of) or upon. I met him AT the elevator, which is located ON the 3rd floor. I looked AT the picture that hung ON the wall. I am AT the store, but you are still ON the bus. I look AT the wall and see the ornamental patterns painted ON it. I hope this helps.

I love the patterns painted on my kitchen wall. Around my kitchen window is some climbing ivy. When I looked through my kitchen window today I saw a man in the park urinating onto a tree.