Answer as a tutorial
Click on the 'CREATE 2D SKETCH'
XZ plane, XY plane, if you are going to create a surface model of a car you will need to use the 3 plane with 3 reference images.... End Elevation, Elevation and the Plane.
Click on the image icon at top right hand side of your ribbon.
This is were you will navigate to your reference image on your computer and insert it in to the plane you have selected.
You can edit the size of your image with the use of dimensions and also constrain your image to the centre point with the use of constraints.
This is how you would go about if you wanted to edit your image.
You can rotate, flip and make the background image trasparent.