This is interesting. I was having the same problem and the examples that the poster is using (the car images) comes from the same Digital Tutors tutorial that had me scratching my head. The tutorial goes through the initial steps of setting up the images in the image plane and then the instructor switches on the grid from the view>grid dropdown and voila!, the grid is OVER the image. This was driving me crazy and I've been looking all over for the solution, before deciding that it was simpler to just drop the alpha of the image. That was until I started getting some seriously strange behavior while zooming. I tried moving the image plane waaaaay back in space and still got some freaky action. Oh well. I'm sure it's a fairly straight-forward solution staring me in the face.