Get a few people to go online with people in the cars that you want in the picture and then set the laps to about 5 and then organise them in the way you want, finish the race and save the replay. When you watch it back, pause and take pictures.
I would like the ability to do this too but don't think its possible in game so only option is editing on a pc which I've tried to do but I lack the necessary skills.
Thats how I got the two pics in the OP of my tuning garage and a few others of me jumping over a Super Bird. If you dont have a friend or sibling around I do reccomend find a way to either hold the joystick down or to hold the circle or square button down.