Speaking as a rider some can get pretty particular about the bikes they ride harley/honda/BMW... like the guys used to be about ford and chevy lol ( ok older ,country girls I know you will understand the ford/chevy thing lol) so be carful with pics.
How about some type of open road theme ? land scape fabrics /trees etc or a panel of the outdoors split into with a road disapearing into the distance etc.
Or if you can get some pics of them together on the bikes would also be an idea . I made a photo quilt to surprise someone once and nabbed all the photos off of facebook !