Board Thread:Questions and Answers/@comment-28500575-20160814021829/@comment-26447088-20160814212414

If you click on the image you want to update from the unit's page (like you would do with the placeholder if the card art was missing), then go to "More Info" at the top right, you'll be brought to the image's details. Under "File History", there's a "Upload new version of this file" link that will bring you to the Upload page.

Doing it through the mentioned steps should make your image appear.