Most Helpful Girl
I think the majority of people feel like that, at least from time to time.
In some ways, a picture is more accurate than a mirror (I'm stretching way back to high school physics, but it has to do with the fact that a mirror creates a virtual image rather than a real image). To give an example, a lot of times people who are overweight don't really HOW overweight they are until they see a picture of themselves. It's not that the camera adds pounds, it's that what you see in the mirror isn't always a good reflection or reality.
But that said, pictures aren't always the best indicator of what you look like either. Pictures are just a flash in time. You're often caught with a weird/unnatural look on your face, for example. It's not that you go around with that weird look on your face all the time, it's just that the camera caught you, say, as your face transitioned from one emotion to another---something that people in person would never have noticed. Or another thing is that people often pose for pictures, which is also putting yourself into a sort of unnatural state. I used to be really down about my smile because I thought it looked weird in pictures, but my partner pointed out that my natural smile, like the smile you can't help but make when you're happy or when someone makes you laugh, is different from a "forced" smile for a picture.
And lastly, keep in mind that what you see in the mirror or in a photograph is often affected by your mood. It's not uncommon for our self-esteems to vary based on internal factors (how happy you are, how good or bad your life is going at the moment, etc. Sometime's we're our own worst critics.) and external factors (like, if someone complimented you that day, you might look in the mirror and think "Yeah, I look pretty good," but if someone insulted you that day, you might look in the mirror and see someone ugly).