The logo below looks pretty blank, doesn't it? Just the word SNOW on a white background?
But there is actually a picture of a polar bear hidden in there. A polar bear in a snowstorm, to be precise.
To the human eye, there is no discernable difference between the pixel values 254 and 255, especially when displayed on a computer monitor. Actually, with the colourmap conservation schemes employed by most browsers, they are probably displayed as truly identical pixels anyway.
However, the difference is there in the image file, even if it is not visible. This allows information to be hidden in an image, a fact that has been exploited by a number of steganography programs. In this case, there is a polar bear drawn in colours one-less-than-white.
To see the bear, we simply tweak the colourmap.