I would watermark them for sure. There are several ways to do that. You can either plant it on each image with Photoshop sometime before you upload them, or you can just load them all into a program that does the watermark for you. If you are building the site through Wordpress then there are a few different plugins that will batch watermark images for you. It is always a good idea to put something on your picture just to keep people from trying to pass your work off as theirs, but you should know that the automotive companies own the trademark and copyright of any picture you take of their products. So unless you are going to scrub the emblems off and any other identifying mark, you really don't own the copyright of the picture. If you plan on your site taking off huge and getting lots of attention then I would just stick with a ghost image of your logo somewhere on the image (like the way Autogeek does with theirs) and call it a day.
I've got a couple pictures that I have to chase down people for every once in awhile. I've got a picture of my son washing a little pedal car from ten years ago and I get five or 6 emails a year from people saying that they have found it on someone's website. Because it isn't a picture of a specific car, I absolutely own the copyright on it. On pictures like that I will definitely send a cease and desist letter and make them take it down. However, on a picture of a great correction on a car that is clearly identifiable with a brand, I don't pursue those as often. It isn't harming me in any way if someone uses my picture. If they suck so badly that they can't get decent results worth photographing, then I guess they'll have to steal from masters like us who can and do get results.

(say that to yourself when someone steals a picture and you'll actually feel proud that your work was good enough for someone to take)
Copyright anything else on your site, though. Any articles you write belong to you. Even posts on a message board like this belong to you (thus you'll see the copyright in my signature on the recommendation of Mike Phillips years ago) Protect anything that you create vigorously.