My time has come!
The above stereographic image is for cross-eyed viewing (most stereograms are wall-eyed, so you may need to put your finger in front of your screen until this one comes into focus)
This is an image of Honolulu, Hawaii, published by NASA. Note Diamond Head (the volcanic crater) in the south.
Here are some other stereopairs published by JPL:
Wheeler Ridge, California
Mount Saint Helens
Salt Lake Valley, Utah
Wellington, New Zealand
Normally it means you picked the wrong distance to the screen!
Tinker with that first - it might be much further or closer than you think, all depending on your screen and eyes.
Also, a hint: the central circle should stay straight in the middle, not closer to one or the other. At that point, below there will be three equally sized pictures, and then you can switch your sight to the central one.