Sometimes I use new submissions right away, other times I will use submissions received a year ago. It all depends on what I'm looking for to provide the best variety on the site. I can't stress enough that photo quality is very important. This doesn't mean you need an expensive camera. What it does mean is that your car is shot from a good angle, with good lighting.
Composition is everything.
I get some fabulous photos of cars in driveways and parking lots, and I also get crummy ones of cars with otherwise beautiful backdrops.