hi those images will work fine..if you have masked the background to just show the car you need to save them in a format with an alpha chanel ...I recomend you save them as a targa or tiff and make sure you choose 32 bit format to include the alpha chanel.
once you have created your images in Photoshop, make a note of the image sizes...Then in maya create a primative plain the same ratio and assign a new material (lambert would be best ) then in the colour attribute of the material click the checkered box on the right and select the file option. In the attribute editor next to image name click the folder icon and browse to your image.
the image should now show on your plain if not then press 6 on your keyboard.
hope this helps....goodluck