By RedNote on Etsy (THE place to go if you’re looking for unusual wedding anything), the card has space for 16-20 photos and the background can be any color. Using photos from your relationship, it would make a very personal invitation or, as a thank you card, is a great way of showing off a bunch of your wedding pics.
As an invitation, I think it would be particularly good if you’ve been together for a long time and want to show the history of your relationship on the invitation. Or, say, if you’re renewing your vows. The cards cost $41.25 for 25.