My girl did the same thing except read my facebook convo with my best mate... she got mad at me cause I'd talk about women like they're objects...

I turned it around on her and made her feel like the bad guy for going through my personal stuff. She needs to learn to respect your privacy or GTFO.

If she don't respect your privacy, she don't respect you. That is all. Shouldn't matter if it's stuff that happened before yall were dating, end of story.

You left you diary/phone open for view lol. Ok not a very big problem, let her know that it was before her and you were together it should calm her down. Then tell her that they were of some crazy chick that sent you the pics some girl that was staukerish and you forgot the pics were on your phone act immberised and delete them say I'm sorry some girls just won't quit. It will let her know she is important and that other chicks want your dick.

_________________
Live and let live...

Tell her:
What was not in your time, let it be.

If those texts and pictures were from before you met, she shouldn't bother about it. BUt think about it, if you were in her shoes, you'd be in the same way. BEcause your mind plays tricks on you, you start to wonder and imagine things that are not real. I've been there.
Don't put a lock on your phone, this only lets out a message that you're hiding something, specially, when you didn't have one to start with, if my GF put a code on her phone all of the sudden, then I would think something fishy is going on.
As other guys have stated, just let her be sure that you're only with her. I don't know how much time you guys been together, it doesn't seem long, but with time.. trust is built, sometimes it takes a lot of time. Sometimes a girl doesn't trust you because she has done bad things in the past, or has actually been hurt.
True trust, and confidence comes with time, patience, and work from both people.
I wouldn't go to the extremes as to dumping her, but I would just make her feel sure about the relationship.