You need to create a file in notepad called '.htaccess' . It must be named just that; no .txt after or anything.
In this file put the following:
--------------------
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^
http://www.yoursite.com [NC]
RewriteCond %{HTTP_REFERER} !^
http://www.aforum.com [NC]
RewriteRule [^/]+.(gif|jpg)$
http://www.mattsbabes.co.uk/leecherspicture.jpg [R,L]
--------------------
Basically the line:
RewriteCond %{HTTP_REFERER} !^
website address [NC]
must be repeated for every site you want to allow the images to be shown on, including site subdomains and so on.
For the final line just replace the URL with your image of choice (this is the replacement 'stolen from...' image)
Save the .htaccess file and upload to the directory of choice (it must be in each directory, placing it in the root won't cover all the subdirectories).
As a further note, if it's for bandwidth issues, make a 1x1 pixel gif as your replacement image. With a big 'stolen from...' image you're negating the point of doing it since you'll still use the same bandwidth.
Hope that makes sense.