Old browsers such as Internet Explorer 8 and earlier had a unique quirk on how they rendered images when they're highlighted. Artists at the time took advantage of that and added "subliminal" images in their art as a bonus for those that use such browsers.
The trick works because those legacy versions of Internet Explorer highlight images using a non-transparent grid pattern, which cancels out one of the two images that have been interleaved into one image using the grid pattern. It dosn't work in modern browsers because they tint images, rather than using the grid method.
Basically, the grid method works like this. Two images are put together like so:
121212121
212121212
121212121
212121212
When highlighted, the "1" image is masked by the highlight, thus this is the result:
#2#2#2#2#
2#2#2#2#2
#2#2#2#2#
2#2#2#2#2
#2#2#2#2#
and the second image can then be plainly seen.
Here is a method of revealing the hidden image using a layer-based image editor like Photoshop or GIMP:
Consult the documentation or Google instructions on how to use layer masks and render checkerboards in your image editor.