You might want to take a look at the bmax source code for loading images:
C:\Program Files\BlitzMax\mod\brl.mod\pngloader.mod\pngloader.bmx
C:\Program Files\BlitzMax\mod\brl.mod\bmploader.mod\bmploader.bmx
etc
C:\Program Files\BlitzMax\mod\brl.mod\pngloader.mod\pngloader.bmx
C:\Program Files\BlitzMax\mod\brl.mod\bmploader.mod\bmploader.bmx
etc