Over the years I've amassed a huge collection of antique cards and images that I periodically use on my cards. One of my all time favorite image is from an old postcard and one I've used on many cards. The best part is that it even comes with a location for added text or personalization. By changing the newspaper headline as I see fit the card goes from "thinking of you" to "get well" to "another birthday makes the news".
I keep the original card in a glasine envelope and each time I want to use it I color photocopy the original on the highest quality setting the copier has. Depending on the look I want to create I copy it either on standard white paper or photo paper. Depending on your copier, you can also scan an image and then play around with size and color. Simple but effective way to re-use a favorite image.