Yeah a bit better. Sorry! Didn't quite follow, but then again once you get to know me I find it hard to follow all the time xD
So, the doctor is in, and this is my honest opinion giving the information (I apologize for length ahead of time)
The blame or wrongness of the situation technically lies with her, but it also lies a bit with you as well. On her side, she shouldn't have gone off on you just cause her brother was annoying her. That right there shows an extreme lack in character and can cause problems down the road in friendship. In other words, she's going to get mad over the little things which isn't right. Sure, brothers can be annoying, I live with a little one that can drive me up the wall at times, but that doesn't allow me to take that out on anyone, even him.
Now with that said, your part in the matter was a bit off. Depending on the situation (and once again this is just with what information is being told to me) if the graphic was something she wanted done badly and you stepped in just to mess with her, that was a tad wrong. A joke is a joke, and I understand that, but at the same time if it was me and my friend and the situation was with a guitar or something I really wanted done, I'd be a little pissed too. Now if that's not the case and it was just a random thing, then really it all lies with her. Once again though this is just based off of what you told me.
Bottom line for me: She isn't that great of a friend. If she's going to go off cause of her little brother, what's not to say she won't go off cause her toothpaste is out? Yes, a little extreme, but I have dealt with a lot of people like that and they are no good, especially the ones that tried to pull the whole "You are so much better than me" after the incident. Trust your gut, if you feel like she was just saying you were better and you don't believe that giving the circumstances, then don't. People sometimes resort to that as a quick 'fix it' for a relationship problem, and it's nothing but an act.
Once again I am no doctor, nor am I studying Psychology. I've dealt with a lot of people in my life and this is just my opinion, do what you will with it. If there is any more to the story or anything or you have a question, ask. I don't bite, I promise xD