A subject (generally a person) is photogenic if appearing aesthetically or physically attractive or appealing in photographs. Photogenic drawing, coined by William Fox Talbot, also describes the earliest method for recording camera images.
The state of being photogenic may or may not necessarily be related to one's physical attractiveness in real life. Models are usually described as photogenic. The bone structure of their faces may represent something that is not generally pretty or may be even unattractive or frail looking, but when photographed, their features can turn into something that is physically attractive.
Two eyes, one lens
There are a few different possible causes for this. First, it is important to understand the difference between looking at someone with two eyes and through a single camera lens. With two eyes, the human brain is able to see the three dimensional aspects of someone's face, even when viewed directly from the front, and it gives much more information than most cameras. With a camera, the subject is viewed through a single lens, and thus much of the three dimensional qualities of the face are lost, and the face may appear to be fuller than it actually is in real life (which is the reason for the expression that "the camera adds ten pounds"), or with different proportions, especially when viewed at a close proximity. An interesting effect can be seen if one compares a close up picture of someone's face to a picture taken from twenty feet away from the same angle (particularly while directly facing the camera). The face will appear different in each picture, and the farther shot will give a better representation of the person's true three dimensional appearance. A more detailed explanation of this concept can be found in the US patent document for the "imaginograph".
Since most photographs have the subject looking directly into the camera, those with less flattering side profiles (such as those with a weak chin or a curved nose) are at an advantage. In real life, we rate a person's face based on their appearance from not just the front, but also other angles, but in most photos, only the front view is available to make the judgment.
Another explanation for the fact that attractive people are not always photogenic is that part of their attractiveness may be due to the charisma they bear in real life due to the way they move, express, and carry themselves. While this will positively influence the subjective appearance of that person in real life, a still photograph usually fails to reproduce these attributes, possibly rendering a picture of the person less attractive than the real-life perception and contributing to classify that person as less photogenic.
The lighting when the shot is taken can also have a large effect on a person's perceived attractiveness. Also, a person's face may look different depending on the angle and intensity of the light being reflected off the face. But, lighter skin tones and features may appear washed out when taken from a flash camera. This effect is magnified when a flash camera is used and may cause undesirable features, such as ridges or bumps to appear more pronounced than they otherwise would.