Why does it still bother me he had a picture of a girl?

It is really getting to me! This was like 2 years ago, but my boyfriend gave me his old phone because I needed one, and he got a new one. So I was looking through it and he had a picture still in there. It was of a girl - I dont know who she was. Well, she was just sitting on the edge of a bed (like in her bedroom) smiling. It was obvious by the picture that he had asked to take a picture of her. At this time, we had been going out for almost a year and he had ZERO pictures of me in his phone - ZERO. When I asked him about it - it was when he called me from work and I was upset and started yelling. He hung up on me because he said he couldnt talk about it at work. Then, later he said- its just a friend of his. I asked him why he didnt have any pictures of me, and he said cause he is with me all the time -or something like that. I told him it was because I’m not pretty enough for him. So anyway, to this day it still really hurts me. The thing is- now he always wants pictures of me. I dont know if its because I improved my appearance or what. He is going through some family problems so I dont want to bother him with something that happened two years ago, but it really hurts me still! Why? Do I need to just let it go? Oh- and I know he would never cheat on me- I trust him. Please Help!! What do you think?