ok i will give it a shot, hope you can follow me:
For Wheel swaps:
select the wheels from the pic you want, leaving a little bit of tire around the edge. Copy and paste them into the pic of your car.
Go to Edit->Transform->Scale to get them the right size.
If you have to, do Edit->Transform->Flip Horizontal.
Position the new wheel over the original wheel, and use Edit->Transform->Distort to get them as close as possible to fitting over the original wheel.
For Lowering:
Select the car like you do for a color change, except just outline the bottom of the car, and highlight the rest of the pic above it. So basically you should highlight the entire picture except for the ground and wheel...hope that makes sense...
Once it is selected click on the move tool (the arrow up by the selection tools), and then use the arrow buttons on the keyboard to nudge the car lower.
Hope that helps
