For myself, I'm inclined to encourage some standardization here. For class abilities, use the picture crops off the iconic character sheets: All the Assassin class abilities get a picture of the assassin on it, for instance. Then choose a few illustrations from the rest of the book to provide a standard quick reference for the type of card (i.e. all the conditions would have the same cropped illustration on them).
Alternately,as Doublebond's said, just don't put art on them at all, or settle for a reference icon
(a blood drop for condition cards, for example), and increase the size of the text for eased reading. In fact, if you drop the art they could go into landscape layout, which makes reading easier as well on something CCG card-sized.
Once Spellbound comes out, I'd love to see spell cards, too, but that may be a case for an actual third-party company.