As an alternate, how might I generate a new pixbuf with the 2nd image's bits replacing the 1st image's bits at a specific location ? (I'm sure Gimp can do this, I'm just curious as to how it manages this...)
gdk_pixbuf_composite() does this. If the source pixbuf has an alpha channel, I believe it also does alpha-compositing on the colors. Here's a URL:
http://library.gnome.org/devel/gdk-pixbuf/stable/gdk-pixbuf-scaling.html#gdk-pixbuf-compositeIf you want to do something fancier, you can analyze and/or modify the pixels comprising a GdkPixbuf. I've used this general approach to do some unusual things with raster images in pixbufs. For some simple sample code, take a look at example 1 in the following URL:
http://library.gnome.org/devel/gdk-pixbuf/stable/gdk-pixbuf-gdk-pixbuf.html - Bob