I've made greeting cards from my seashell photos. Here's my starfish birthday card (sorry for poor image quality).
I usually do the text in blue, but I was out of blue edged blank cards.
My thank you card has a photo of a queen conch shell, and my congratulations card has a sea fan photo with caption, "Congratulations from Your Biggest Fan!"
I use deckled Strathmore blank cards, because they give a more elegant look, I think. The cards cost me about .50 to make last time I bought blanks online.