the best one I have seen had a picture on the side of a cleaner with wand in hand in the process of cleaning a carpet.
It looked liked something they had scanned from a brochure, like Prochem, and then had a signwriter copy and transfer to the van. Really stood out.
However, most decent sign writers will be able to show you how it will look on your van before going to print, so that you can have a play around with images and colours etc.
Hope it helps.
Dan