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The "white" pixels in the apple picture contain the picture of a pear, stored at a much higher intensity, i.e. very bright. The "black" pixels in the pear picture contain the picture of an apple, stored at a fairly normal intensity, but scaled down to near black with the gamma correction. The image contains a gAMA chunk specifying a file gamma value of ...

This was a little too much for a comment, but hopefully it helps. So, I am fairly certain that this issue deals with the way the browsers interpret gamma information with PNGs. It's a pretty fun problem and deals with the ambiguities of gamma information in the first place. This article: http://hsivonen.iki.fi/png-gamma/ provides a very nice summary of the ...

Changing the gamma of an image consists in modifying the value gamma in: (R',G',B') = (Rɣ, Gɣ, Bɣ) which gives the output pixel color (R',G',B') displayed on the screen after applying the gamma function to the initial pixel values (R,G,B) (considering R,G, and B normalized between 0 and 1). Now, let's take the red channel for example. If R ...

It's not rounding pixels and the ICC color profiles are not the issue. It's a trick image, and some browsers display PNGs without gamma data. For those browsers, you see one thing, and for other browsers you see the full image (with the pear hidden in the background). I see either a apple/pear trick image, or I just see the pear, depending on if the ...