When i was doing Motorsports Photog it was always an issue being ripped off. Watermark it, keep the resolution low (800x 600) or so and on my website at the time I could protect the image from being downloaded. They could do a screen grab but that makes for a poor pic anyway.
www.snapfiles.com has got some free watermarking software which I would ALWAYS advise people to place on their pix. I used to put it in some awkward spot, radiator, bonnet etc so it was difficult to clone or crop out. Got ripped off by a Rally driver over here who used my image on his promotional brochure and never paid up.
My advice is to watermark everything, just in case.