one way to do it...
1. take 2 pics...ideally camera should be on a tripod- 1st pic with bonnet up, secondly 1 with bonnet closed.
2.open both images and place one on top of the other..ideslly you want the pic with bonnet closed to be the top image.
3.add a vector mask, select the brush tool and ensure black is selected.
4.Simply brush through what you want to show through from the underlying layer....this should just be the engine bay.
5. flatten and show off to the rest of the world.
Hope this helps.