Redirect image requests.

[CPURules]

Ok, here's what I want to do.

I have a website set up, and I'm concerned about non-existant image files. I think what I want to do, can be done with .htaccess
Anyway, my image directory setup looks like this:

htdocs
-images
--138279.jpg
--153968.jpg
--167833.jpg
--183219.jpg
--notavailable.jpg

The .jpg files(apart from notavailable.jpg) are all pictures I've taken. If someone tries to access a picture that is NON-EXISTANT(and only the jpg files) such as 195823.jpg, it keeps the same URL, but notavailable.jpg is displayed.

Examples:
URL Entered: http://site.com/images/138279.jpg
URL Shown in URL Bar: http://site.com/images/138279.jpg
Effect: Shows the picture

URL Entered: http://site.com/images/195823.jpg
URL Shown in URL Bar: http://site.com/images/195823.jpg
Effect: Shows notavailable.jpg

If this is even possible, please let me know. I will +rep for your help.

TIA.
-Charlie