I'm also a detailer on the side although mainly for my own car. In my opinion, you need to get some straight on shots with the sun hitting the panel to prove it's clean.
I did an experiment on this and almost any car can look great in the shade after a good wash/quick wax. Getting it in the sun for the pictures proves that you can remove the swirl marks and imperfections in paint....especially on a black car.
This is a Mustang I did awhile back. When I examined the car in the shade it looked pretty good and I didn't think it would take much work.
When throwing it in the sun or under the halogen lights though, this is what I found
During the detail I took some 50/50 shots
Then after the detail get some shots under the lights and in the sun to show the final outcome
I've found these before/inprogress/after shots to be extremely useful to show customers what can be done with their vehicle and really helps show off how good of a detailer you are. Hope these ideas help.