Please take the time to read this.
When requesting:
Shots of your car greater than 640/480 pixels
Decent lighting (colors not washed out, nor too dark to see)
As little grain as possible
and most importantly, GOOD ANGLES.
For example, if you want to see how wheels and suspension will look on your car, don't post a shot down on the front driver's side corner from 20 feet away. You won't be able to tell a damn thing.
I know it seems simple, but for the following "common" requests, these are the best angles:
Wheels - Side
Suspension - Side
Color Change - Any angle that shows a lot of paint is good, providing the light doesn't wash it out.
Shadowline - Side (though this one is easier to do on a less-desireable angle than wheels, for example)
iS front Lip - Front
Body Kits - A good shot from the front, the side, and the rear. If you want something "pulled" off another car, and stuck on yours, try to match the angle of the other car.
Keeping those above details in mind, will greatly enhance the ease, and realism of whatever photoshop that's done for you.
SIGNATURE PICTURE SIZE RESTRICTIONS: 600x225 pixels
AVATAR SIZE RESTRICTION: 80 by 80 pixels or 19.5 KB (whichever is smaller).
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Here is a quick way to learn all those things you want to do in Photoshop.
http://www.photoshopjunkie.com/ Head to the Tutorial section.
Here you can find a good idea of how to lower your car, and other things. Remember, it takes time, patience, and a lot of practice in Photoshop to learn how to pull off a car chop correctly.
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Become a master, and you can pull off stuff like below:


I did this with Photoshop 5.0 ages ago. Think of the possibilites of the new CS2, and what you can accomplish.