Improve error message more.
Check that 'image' is bound to an image.

I would prefer to accept the student's image as an expression in the
definitions window, but I don't know how to access the result of
evaluating such expressions. Unlike DrRacket, the sandboxing seems to
throw away the result of naked expressions in the program. In future
editions of the course, we should adapt the first lab session to
teach (define image ...) if we still need it in the first homework.