These are loaded from a MOD folder (activated) and every reference to any images must be declared in a "new" RaceConfig.xml file where the exact data links anything to supplemental (or through renaming of default assets) elements.

Yes, I have done it as Ledzeggelin says. I've never messed around with mod folders, thought that may be another way to do it.

Considering this 'question' was actually into a *Modding* forum thread, i instinctively presumed my answer could be -somehow- to the point.

But, there ARE many different ways and locations to simply add Logos & Portraits  - indeed.

As vague as the intention might have been expressed by the OP, one may want to use new stuff or replace defaults altogether straight from the usual 'MyDocuments/...' folder.