You can select a Card image by clicking on the camera icon, and manually setting it. Otherwise, the first image that you attach to the Card will automatically become the main Card image. You can always change it if you want to select a different image.
Click on the camera icon to add a Card image.
The "Change Card Image" dialog will open:
From here, you can either add an existing image from your Lumi project, upload an image from your computer, or select an image from Unsplash.
Once you've added a Card image, it is also stored on the Media tab, as indicated by the "1".
The Card image will display everywhere you use the Card. For example, on this Location board.
You can crop, change or remove your Card image by clicking the camera icon again.