Any card can have an image added to it. Either add a link to an image, or simply select the Google Drive icon, add a JPG/GIF/PNG image to your Google Drive using the Upload function and then save the card. Workspace even accepts animated GIFs!
The image ratio used is 2:1 for our Workspace card thumbnails. So something like 800 x 400 will work great because it is using the 2:1 ratio