The ATP isn't that convincing, though I don't have a lot of experience in assessing this from pics, so I don't know how big it is suppose to be.
And like Wes said, you don't have a lot of fat on your body at all. If I were you, I'd start trying to get in a calorie surplus.
I assume the two first pics are taken in a natural position for you? You can see your shoulder blades sticking out and they are protracted. A lot of upper back work could fix that. Also, working the serratus anterior muscle will help rotate the shoulder blades back to where they should be. The pointy end you can see sticking out, that's the bottom point of the shoulder blade, working the serratus anterior can help pull that back in. Also, working the traps, specially the middle and lower fibers, will help.
Serratus exercises: anything that protracts (pull forward) the shoulder blades. If you do pushups, you should do push up-pluss. It's just a push up where you protract the shoulder blades at the end of the movement. Or you could do only the "plusses", if you don't want to do pushups.
To work the middle traps you can start of lying face down on a bench with your arms hanging down 90 degrees out from your body and then bring them backwards. You can probably try this without weights at first, try to feel the muscles in your back working. For the lower traps, do the same only not 90 degrees but more in front of you, but a little to the side. I'm off to school now, but I'll see if I can't find any articles explaining these better when I get back. Shoot me a PM if I seem to have forgotten.
Now keep in mind, I'm not a healthcare professional, the best thing to do would be to see a physio, but not everyone wants to/has the time and money to do that. IT seems your willing to take advice about these sorts of things off the forum, so I thought I'd give