Opening line mentioning how much of a newb I am.
If I want to add a card to a game, do I have to edit the plugin as if making my own plugin, or is there a quick way of adding cards? If, for example, a new series of cards comes out for...Yu-Gi-Oh! or something, and I want to add them, do I have to edit the plugin, or make a new plugin?
It is best to wait for the plugin's maintainer to add the cards from the new set so that everyone will be able to have the same cards. If you just add the cards yourself, no one else will be able to see the card images and you could cause other problems with the program. If you notice a new set has been out a while, but the plugin has not been updated, you can contact the maintainer through these forums.
On the other hand if you were wanting to make a new set of cards all on your own to play with just a small group of friends, you could just copy the plugin, rename it, and change it to accommodate your home-made cards, then share the "new" plugin with your friends.