If you can see the bonnet or top of the boot or roof on your image it's not suitable. The image has to appear 2 dimentional. The best way to do this is to ensure the camera is at the same height as your door handle. check out the photo taking tips here

What size should the image be?

The minimum image size is 640 x 480 and Maximum image size is 1600 pixels wide, however slower editing will be experienced with images of this size. Ideally an image of about 1024x768 provides the best results.