You don't need to draw it to save part of it, and can avoid making it an image.
How about:
LoadPixmap() to load the whole image
CreateStaticPixmap() to define a static pixmap within the pixmap you loaded
SavePixmapPNG() the static pixmap
How about:
LoadPixmap() to load the whole image
CreateStaticPixmap() to define a static pixmap within the pixmap you loaded
SavePixmapPNG() the static pixmap