Root level override on content block?

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Hi all
Just wondering if anyone has done a root level override on a content block.
Basically I have CDN running to serve my sites images, and I want the block to automatically pickup the CDN URL instead of manually updating the hundreds of images the site holds through the source view on the text editor.
Thanks, Simon

 
blueorangedesign replied on at Permalink Reply
Sorry should have mentioned this is specifically for C5.6.