Not liking photos of ourselves has much to do with symmetry and the fact that we are all (some more than others) asymmetrical. I look in the mirror and the blemish appears on the right side of my forehead, I look at a photo it is on the left ...it is "uncomfortable" to me. You look at me and you look at a photo of me it is the same. Try it, take a digital photo of yourself, flip it, it will look like what you see in the mirror. Supposedly the less asymmetry the better looking the individual
Additionally most people are our their own worse critic. We know what we CAN look like. We see ourselves only a few times a day. Those we are with see us all day long, not just as we are looking in the mirror. As someone who is not just a few pounds over weight it is a reminder of what I perceive as my short comings or inadequacies.