Go with nr. 3 then, the readme will give you the steps which are literally 4.
http://mxcl.github.com/homebrew/ is an alternative too.
But why do you need libpng if you've never installed anything via terminal nor are you a programmer?
libpng is not going to do what you think it does. It is a library for programmers to use to be able to manipulate (or create, or decode for display) png images. However, the important point you are missing is that png is a normal image type (just like gif or jpeg). Preview (or Safari, or Pages, or QuickTime, or any other program that can view images) will happily display them for you.
The PNG images found on the iPhone include extra data before the PNG header and compressed image data without the traditional headers and footers. Also, the red blue pixels are flipped (BGRA instead of RGBA)