<?
   $im = imageCreateFromPNG("large.png");
   imagePNG($im);
?>

Once large.png is above a certain size, the image is not loaded past a certain offset.

From what I have seen though, if you pipe it out to a file imagePNG($im, "out.png"), then it is a complete file.

I haven't expiremented with it enough to figure out which has the limitation. It would seem that image* does, but you can imageCreate(x,y) the same size image and the image does not get truncated on output to stdout.