Consider :

  Image1.Picture.LoadFromFile('pink_bars.png');
  black := TPngImage.Create();
  black.LoadFromFile('black.png');
  (Image1.Picture.Graphic as TPngImage).Canvas.Draw(0, 0, black);
  (Image1.Picture.Graphic as TPngImage).Modified := True;

Here, the contents of black.png won’t appear where pink_bars.png has transparency. Which makes sense of course, but in this case I’d like the transparency info of black.png to be kept in the final result (for the portion of the image it is drawn upon, here it doesn’t cover it all). I’m not sure of how I could do that short of overriding information in a loop pixel by pixel, manually.