In my experience as a Canvas admin, I'm going to guess the image in incorrectly linked to an old course. Acting as the teacher, it would look fine to you, because you'd have access to the old course. For your students (or when using student view), they'll see the lock because they don't have access to the old course. The most straightforward way to fix this would be to edit the page, delete whatever image you see in the place where the students see the lock, then just re-insert the image and save. You could also try to get fancy switching to html editing mode, but unless you're a web person I wouldn't recommend that route.
Hope this helps more!