Although our wedding had sort of a cycle theme (well, we used the bikes for transportation), the invitations did not.
But maybe that will give you an idea.
We had a few "engagement pictures" taken by our photographer, but basically any friend with minimal talent will do. I told the photographer I needed a picture that would work well in a 4X8 landscape, with us appearing on either the left or right side.
I then designed our own 4X8 invitation card using Adobe Photoshop Element. One side was the chosen photo with the text of the invitation ("Together with their parents, YoungMan and YoungWoman invite you to the celebration of their marriage, date, venue...") and, on the other side, were a few more details about the wedding (a map, the addresses of the ceremony and reception venues, how to RSVP, etc.). I have no design experience but it's not really hard, except for the "(%* map.
I got them printed by Vistaprint (online) but there are other printers out there that can do that sort of stuff.
I like to think that it was as good a keepsake (if not better) as regular invitations. Our thank-you cards after the wedding were similar, with a collage of photos from the wedding.
I will send you a PM with a link to see the results.
Enjoy the planning process! It's only as easy as you'll allow it to be....