Need simple help please!

I apologize if this question is a bit plain but i don't have much time to read maunals and stuff but i would really appreciate it if someone here can answer this question for me.

I was wondering how do you edit the start menu button. I want for the diamond skin specifically, i want to change the blue circular logo button, and i know how to do photoshop, but i can't find the img file to  of the button or find a way to edit it in skinstudio.  Help please and thanks!