I agree. There are some complications which set in, when you reset the background of a window that is already mapped to the screen. I noticed that if the dimensions of the render_to_drawable is not exactly the same as pixbuf dimensions, x-y errors would occur. These can be avoided by some logic to get the pixbuf x and y size, etc. But that brings up the problem of what happens if you resize the window? Will the image resize? How would you tile the image? etc.

I was thinking about it last night, and the gtk c-libs developers probably didn't want to bloat the code with all the logic it would take to enable swappable background images. So we see it can be done, but the provided methods of using the styles and xpm files, is easier, and will probably work well for most applications.