Emails: Override Dynamic Header and Footer Image

Would like to know where the config for headerImage and footerImage is located within the Sisense dev UI. Created new branded emails, held within the branding folder, but the images are not rendering in the test emails. 

Example of the original headerImage token: 

Seems that there is a locals directory or mapping stored somewhere but am unable to locate it. Using the same cid:<filename> for using the relevant PNG file from the images directory within branding.

There's no documentation that explains this.