For games that feature, in some way, the revealing of an image. Most often as a reward for completion of a segment, though some of these games use revealing images in a manner that crosses from theme to pattern (ie: key mechanic). The theme is most common in erotic games.
Nonogames feature revealing images, not just as a reward but, as part of the solving mechanic.
Most Qix-alikes reveal images as a reward for completing a board. Most interestingly the Gals Panic series of games use it also as part of their mechanic: players must capture a certain percentage of the image to be revealed, rather than a percentage of the full playfield.
A feature in some Qix-alikes where the image to be revealed will change to something unattractive if the players performance is not adequate.