You can set a card cover in the Placker board view. When an image is added as an attachment, there will be a button "Set as cover" next to it:

This will make the image be shown at the card front in the board view:

Alternatively, you can click “Cover settings”, then select an image or color, and the light or dark theme.

You can also set the size, so that the image overflows the card title with a gradient, or that it is separate.