CDN for my thumbnails ( what replacement variable needed?)

[dog-tag]

Hello,

I've gotten abit of out my depth here.

My CDN works with smilies, avatars, attachments etc but it doesn't work for my thumbnail images.

This works for attachments -
Search for Text: src="attachment.php
Replace: src="http://c.mydomain.com/forum/attachment.php

That works fine, but when I have a thumbnail it doesn't load the CDN URL. What is the variable that I should be using?

Thumbnails just have Thumb1 or something like that at the end, so I'm surprised its not working for me.

G