Check out Enzo's avatar. I think something like that would look good on the back of a shirt if it could be reproduced clearly.
Would there be any problems with copyright?
Enzo's avatar looks a bit 'busy'. And I don't know about the copyright. If the picture was from an original by a CA member, then I can't see any copyright problems.
The suggestion from Martini & FS a while back was to have a photo of a recognisable part of the circuit (a view up the pit straight was suggested), and photoshop it into a simple line drawing. And as Steve has said above, the more colours, the higher the cost.
When considering ANY design, bear in mind there may be a physical limit to the size of any picture. My local contact reckoned that the 2-beers pic last year was right on the limit heightwise.
Following on from Lang Tall's suggestion re flags, I think we should drop the Union Flag from the sleeve, as there are many different nationals who buy/wear the shirts. At the very least, if a flag was required, it should be the European flag.
Keep the suggestions coming - they will be helpful to Martini to show him what we want.
Del