I think that is the confusion here - people are not 'uploading' images to Pinterest, in the main.
They are using a link button. Pinterest may create a cached version of an image, but the users of pinterest are not actually saving a copy of say my map, then uploading a copy to pinterest.
That's just not how it works.

There could be a few random people who do that... but the bulk of any CG things that show on Pinterest are actually linked back to the Guild.
Not in the most ideal fashion, no. But they are linked back.