It compiles on Linux. Only a few warnings:

Code:
libpng.pas(1629,30) Warning: Mixing signed expressions and longwords gives a 64bit result
libpng.pas(1635,44) Warning: Mixing signed expressions and longwords gives a 64bit result
libpng.pas(1642,30) Warning: Mixing signed expressions and longwords gives a 64bit result
libpng.pas(1648,47) Warning: Mixing signed expressions and longwords gives a 64bit result
But I wouldn't have a clue how to test it. Those libpng functions look pretty complicated. It would be nice to have a demo program, or a simple-to-use class that wraps everything up.