i think most of us have been playing GT4 since release and are pretty accurate on the racing lines..... practice, practice, practice.......
i don't know about everyone else but i usually just go for a spin or have a good race, and when watching my reply if something good comes up that i think is worthy of a photo, just save the reply, head to the replay theatre and when u play it over, you can hit select to take photos.... this is also good if you want multiple car shots.
if its really a big problem with positioning, just drive around and park your car at a spot were you would like the photo, i personally prefer action shots with motion blur, as evident in most of my photos...... see below......