While searching the iphone´s files i've noticed that almost every .app folder has several .png. I've changed a few and changed the GUI of some apps. But for example there are some that I just can't open (For example: MobileSafari)
I use PC so I was thinking that maybe that's the reason.
Can anyone please confirm if you can open them in OSX?
Not sure your explanation is clear, but if you are attempting to edit existing PNG files, they are (often) in a proprietary iPhone format. iPhoneBrowser will convert them to Windows compatible (not exactly standard) format when you copy them to your PC.