Yes. I see that does work. If I upload a template the URLs are changed, but if I replace or add a new photo using that email template those URLs are not changed to the CDN path.
If there's an option to use the CDN for email assets it should cover all cases. Customers are going to upload email templates and most likely switch up photos. They will also, and perhaps more likely, create new templates from scratch using the WYSIWYG editor or the builder. At present it seems like the CDN feature is not complete. The only way you can use a CDN is if you manually update the URLs or upload a email template that's doesn't need any further edits.
A CDN for MailWizz is very important in my opinion being that its a self-hosted solution. When you send out a campaign your server's going to get slammed with requests for assets. Using a CDN is simple solution to this potential performance hit. MW should make it simple to use a CDN. It seems like it's almost there, but not quite yet.