Car photoshot composition

[denzjos]

One option is to draw a path around the car with the Paths Tool. Adjust the path with the handles and if satisfied, convert the path to a selection ( gimp menu : Select / From Path). Then you see marching ants around the car. Invert the selection (gimp menu : Select / Invert) and press the delete button on the keyboard to delete the surrounding. Then you can load a picture in a new layer and place this layer under the car layer.
   
Silly composition, but only as a demo