Another way to think about “unfocusing your eyes” is refocus your vision on an imaginary object that is slightly beyond the image (in this case behind your phone). This is why people say to move the image slowly away from you while relaxing your eyes but it’s unnecessary- all you are actually doing is widening your pupillary distance to achieve the optical illusion, which is what naturally happens when you focus on things at longer distances.

Look at your phone, then look at something past it and you’ll see your phone doubles in your vision. Just do that to the picture until you see a third vertical set of photos. The effect is not very 3D in this case as it would be with a magic eye but it’s slightly more dynamic than just looking at the screen.