Using colours is great, but sometimes I forget which colour is used for which card because I use them for different purchases (food, fuel etc). So being able to apply an image just like you can with pots will be great, even better yet, if the virtual card is linked to a pot that card takes on that pots image!
Unfortunately, this isn’t possible. Apple Pay/Google Wallet only allows a set number of pre-defined appearances for cards, and so that’s why they have to look like they do with a restricted option list.
Additionally, this is why wording or pot names can’t be added.