Best way is to use fraps. With fraps you can strart grabbing shots with one key press by assigning and pressing a key on your keyboard, you can set it to take screens every 2 to whatever seconds you want while you play or you can watch a replay and take them individually. You can take pics individually while you're driving by having the keyboard in your lap but would be a bit silly, no?
Fraps just grabs whatever is on screen that moment, if you can steer and press a key on your K/B at the same time then go for it. The free version will take pics in BMP format which results in a large file so you'll have to save them into jpegs by using MS paint photoshop/GIMP.Irfanview
You can also press ctrl and printscreen and paste this into MS paint or photoshop/GIMP.Irfanview but this takes a while if you want to take more than handful.
Steam and xfire have screenshot apps but last I read on steam is that it doesn't work for Dirt 3, no idea if its been fixed.