The problem is that these images are too large. Choose one that is orizontally short, open it in an image viewer, then zoom out (yes, make it smaller).Now look "through" the screen as if looking at something in the distance.
Basically all 3D imagery is based on showing your left and right eye a different image: 3D movies use glasses to make each eye blind to the image it is not meant to see.
Stereoscopic images instead show both images in a way that you "ignore" the part of image your eye do not need.
If you focus on the screen you just see the L and R parts of the image together, but if you focus on a different distance the images from your two eyes should "decouple" and you will see "double". If you look "far" your eyes diverge and the two images get more and more detached, until the L part of the image you see from your left eye matches the R part of the image you see with your right eye. The result is that you see 3 images: the side ones are "plain" and you are seeing each one of them with only one eye, and the middle is 3D, since it is actually 2 different images.
The problem is that the larger the images are horizontally, the more you have to diverge your eyes to actually match the L and R images. Some people can diverge eyes more than others, and the easiest way to mitigate the problem is to zoom out: the image become smaller and you can manage to "see" it more easily.