[chinoh81]

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?

[NetMage]

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.