Pretty sure a simple suggestion of "don't think of an open lock" would do the trick. Or showing an animation of a lock opening. Even if the required image isn't

Pretty sure a simple suggestion of "don't think of an open lock" would do the trick. Or showing an animation of a lock opening. Even if the required image isn't of a lock, but of a "password image," the suggestion of "don't think of your password image" will probably conjure it up for most.