Add a prefix to certain URL's when parsed

[sinner]

Basically I need a mod to allow me to enter certain URL's in a list (say in the control panel) that, when entered in a post as text, would always put a prefix (once again, defined in the control panel) before the URL when it's parsed. For instance:

http://www.pictureURLhere.com/picture.jpg

Would parse as:

(prefix)http://www.pictureURLhere.com/picture.jpg

But only if pictureURLhere.com was in my list of domains to insert the prefix into.

Does that make sense? If so, how hard would something like this be to do?

Thanks.