That (well, ALL of them) turned out really well. I'm envious. I was going to guess decals on the mud flaps, but that's actually a good (and less expensive) idea with just printing them. I'm so used to needing decals to do such things (with all the countours etc that need to be conformed to) that even on a flat surface it probably wouldn't have occurred to me to just print them out and use paper like that.
That's really the best thing about this forum; all the ideas that keep floating around and keep pushing us outside the boxes of the things we're used to doing.