For those that are proficient in using photoshop or other image editors you can still cut your car out in your prefered program. Just save the image out as a png with alpha channel at a recommended pixel width of around  800px then load that image directly into the background removing tool and click "Save" no need to do any editing.

Minimum size for car cut images is 640x480 and maximum is 1600 pixels wide.

You can also use photoshop to do the wheels as well but we have found it's easier and quicker to use the online background removal tool. If you wish to still use photoshop to do the wheels the resulting image needs to be 110x110 in size and again a png with alpha channel