That is an excellent question! People can look totally different in each and every picture. Some people look different depending on which side of their face you are looking at which is normal. But it's weird how some people look like a 2 in one picture, and then like a 7 in another. Makes me wonder if they've had work done, lol. Anyway, as for how much you can rely on a photo, I'd say very very little. You can't factor in the way they carry themself which may add or tak away from their physical looks or attractiveness.
Of course they can, some people are photogenic, some aren't. Photos can often make you look bigger than what you actually are and define your flaws. I learnt in media class that this all depends on lighting and angle. You might not think about it but it takes a lot of hard work to get a good shot.
Yes,they can actually... I look different in pictures,I've had people look at them and tell me that I look nothing like I normally do. I actually think I'm pretty ugly in pictures, just because I think they snap the shots at the wrong time..and I end up with a dumb expression on my face. Other times I do look like myself, other times I look like someone else.
I feel like my pictures don't do me justice, because I think I'm a much better looking man IRL. Women often take pictures with certain lighting and positions that they can make themselves look much more attractive than they might in real life (kind of scary actually).