the method i'll be using first is using rice paper (or photo paper, or similar; the key is to use the ink from a LASER printer, such as many home-printers like laserjet), same stuff you use on most surfboards. when you wet it with epoxy/riesen, the paper becomre pretty much 99 transparent and you can't see the parts that don't have printing on it. the sweet thing about it? you can run it through a laser printer, and the paper won't smudge or anything when you lay it up. i'm planning to just buy bulk rice paper, printing my graphics on, and then just putting my clear topsheet over it when i press.
also, my buddy and i are pretty artistic if you give us a can of spray paint. i think we're gonna try to make some designs straight from the can to the paper and lay it up under the topsheet. a few months down the road, i'll see if i can remember to post here and tell how it went.
other than that, putting graphics directly on the back of the topsheet costs a bit more money for other people/businesses to do it for you, and it's more of a hassle to do it yourself (or maybe i just really suck at screen-printing?). anyways, the rice paper is a good bet, and it's worth looking in to! it produces surprisingly clean results.
cheers.