I'm afraid our card layouts (whether using a MOO Design or Design Your Own) are not that flexible. We're working to improve this functionality, but in the meantime each layout is locked.
In order to change the amount of image slots available you'll need to switch to a different layout. You can scroll through the available layouts at the bottom of the page when designing your cards.
Please note, multiple image slot layouts are only available on the variable/back side of the cards and cannot be combined with text entry fields.