This happens to me all the time; the prespective-view of the image just messes itself up.
I do not know how to fix image planes. All I can say is "no worries;" as long as it looks perfectly fine in the straight-on views (top, side and front), don't worry at all about the perspective.
A longer alternative is to create a Polygon plane, or a Polygon Box, and scale it to the appropriate size, assign a material to the Box/Plane face, and map the color of that material to a File node, and with the File node, import your car.
If you chose to do this, however, be sure to "physically" scale your box/plane to the right size! You car/whatever will come out looking funky if you don't.
Happy modeling ^_^