Best viewed in full format, in my opinion. Though you can try resizing the window or moving away from your screen if you have a hard time crossing your eyes this far.
Cross your eyes until a third image appears in the middle. Focus your sight on the middle image and behold! 3D! It will be blurry and hard to focus at first, but don't give up! Try to align the dot above the image. Remember to keep your eyes completely horizontal.
NOTE: If it hurts your eyes or whatever, I suggest you stop. It may not work for everyone.
I felt like giving 3D a try and decided to do it with this image. I think it turned out quite well for a quick test.
My eyes feel all goofy now from crossing my eyes so much.
MLP:FiM is © Hasbro