one of example can be just switch from 1 to 100 where 1 is cat 2=image of obama 3 - image of car, and so on to 100.
so in your mind you would have to switch from 1 to 100 but only images, so it would be as fast as you can visualize cat, as fast as you can obama, car, and so on. so 100 images would take some time, and later again and again, more images - less boring. i just did it, for my images, and i noticed that if i am sitting in comfortable sofa with eyes closed and relaxed, after 2nd time of 400 images i stopped to feel my hands, i could not locate them where they are exactly....