by Mary Cooch.
You are using 1.9? Then a possibility is that those pictures are in a course the students are not enrolled in so they can't see them but you as admin can see everything because you can get into all courses. If you get the URL of the course where the pictures are in - and then right click on a picture and get its URL , see if the course ID is the same. If it is different then that could be your problem.