To add a main thumbnail image to your Card  press the upload media button in the top right corner.

Then either drag the image to upload, choose an image to upload, or choose an image within Lumi (ie an image that has that already been uploaded to Lumi) to link to the card.

The first image you upload to a Card will become it's thumbnail in the top right of the Card, which is displayed everywhere. Or you can simply click here to add, crop or remove an image. 

Hot Tip: All images can live on multiple cards at the same time, so if you update/rename them, they update everywhere.... so no need to duplicate/re-upload them.