placehold.it is a great little service which makes you think why didn't I think of that before. I found it via Twitter Bootstrap, bascially it allows you to quickly drop in some placeholder images by specifying a few attributes such as the image on the left which was created with <img src="http://placehold.it/150x150&text=Hello world" />. There is more to the service such as serving up specific file types and changing the colour of the box etc.
Check out http://placehold.it/
Of course you can roll your own version with CFML, check out Andrew Mercers blog post if you are interested: webonix.net/post/cfml-image-place-holder-custom-tag.