Every person can become photogenic. Whether one is photogenic has nothing or very little to do with appearance, but rather with naturalness. People with natural facial expressions in photos automatically look more photogenic than those with posed poses and cramped grins. Practice natural facial expressions and make yourself photogenic.

When you pose for a photo, you automatically try to smile. However, most people reproduce a smile based on how a smile looks, not how it feels. You can't fake a real smile, so if you want your facial expression to look real in the photo, you have to feel it.
To get the right feeling for the facial expression you want, you need to imagine the situation. If you want a natural smile, think of something that makes you smile and makes you happy. Feel the joy. If you only imagine what a smile looks like, it becomes artificial.

You can practise facial expressions in front of a mirror or webcam. Observe how your face changes and how it feels when it's right. This will give you the best feedback and allow you to work on improving.

Once you have mastered the facial expression, you need to work on the timing. It's almost impossible to hold a natural facial expression for several seconds, so it's important to put it on at the right time, just before the shutter button is pressed. The timing is different depending on who is taking the photo. However, you can practise it yourself with the timer so that you get a feel for the right time to smile. The ideal time is half a second before you press it.

Celebrities learn to be natural and photogenic in the same way. They may have the advantage of professional help, but the end result is the same. Practice makes perfect, you don't need to feel silly when you practise smiling in front of the mirror at home.