By "go back" i'm assuming you're referring to closing a full-size image. I've found three ways they can do this. They can press ESC. Click the image itself. Or click in the negative space surrounding the image excluding the "Previous" and "Next" regions.
Doesn't really seem like a huge issue since there are multiple ways to accomplish the objective, though I suppose a small "X" could be added if you wanted a visual cue.