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This add-on works well, I believe.

 

Allows you to set images to be viewable by guests.

 

I've not used it myself so I don't know how fine-grained the permissions are. e.g. what if the image is attached to a private forum post, etc.

 

http://xenforo.com/community/resources/tinhte-image-attachment-optimization-cdn-support.909/