Automatically adding images to posts? – Not simple :-(

How can I automatically add a default image to an image-less post?

All I’m trying to do is actually automatically add images to posts via a default image.

I’m using rss poster which converts articles into posts, and searches as well as posts images from those articles when it finds them.

But..

when it doesn’t find the images, I want to use a default image.

So how can I automatically add a default image to an image-less post?