Translation:She is looking at the picture on the wall.
Four months ago, I wrote:
I would say SUR. I think it's an odd sentence with ĉe.
I still think this is the case, but I should add -- that ĉe means basically "close enough to touch" and you'll occasionally see ĉe used this way. We should underscore that sur la muro does NOT mean "on top of the wall" and seems to be by far the more common expression for this meaning.
At the same time, we'll see lots of people standing ĉe la muro or similar, so this expression is fairly common too but usually means something slightly different.
A thing can be at the wall (close to it, next to it), while not being on the wall. For exaple when a picture is on the floor, leant to the wall, or it's enough just thinking about a piece of furniture or a person next to the wall. Now we cannot make this distinction artlessly in Esperanto.
Edit: Actually... Can I say "je la muro" for "on the wall"?