Especially if can't see the picture.

If it helps to know, the way they work is because each eye must focus on the page (or screen) but your two eyes are not focussed on the same spot! For a parrallel-viewing image like the one posted, find a repeating pattern element, move fairly close and let your focus slowly relax (it will probably help to make the picture bigger). You want the two copies of the pattern element to move towards each other as you relax. Once they meet, hold your eyes' divergence there and try to make your brain believe your eyes are seeing the same element - tilt your head to make them match, untense your head and eye muscles, etc. When they lock, it should come into focus and the pattern will look like it's floating inside the page or screen.

The very first time you manage it, you will find great difficult in looking around the rest of the picture whilst keeping your eyes' convergence right. Try not to blink - you will probably loose it if you do! It will gradually get easier, but it takes some time. The picture posted is not a difficult one, fortunately. Use a pattern near the top of the image to match up as that is background and will make the rest of the image easier to see.

Wade.