I do want to share my useful BeRoPNG unit here. It's very tiny but complete PNG loader with a own tiny inflate routine, so it has no external unit dependencies. It supports even extended features such as Apple's proprietary extension CgBI and so on.
And here is the link: http://rootserver.rosseaux.net/stuff/BeRoPNG.pas (36 kB src)
It's licensed under the 2-clause simplified BSD license.
So have a lot fun with it