For modern day matte paintings, artists use everything from photos to 3D CG to painting. Nothing is out of bounds unless you don't have the permissions to use a photo someone else took. Usually getting the shots to look photoreal is very important and how you get there doesn't matter to your supervisor.
I see these illustrious buildings being presented on here that I say to myself..."There is NO WAY they could have painted that!"
Complicated things like a moving camera showing off amazing details in complicated buildings with realistic metal and glass reflections that glide across the beautifully textured surfaces, etc, etc, can be done in 3D. If you want to get into matte painting and be assigned some real cool shots, I'd say you should learn how to work in 3D (MAYA, MAX, whatever). Your background as a painter will probably help your artwork "sing". In the end, a good looking portfolio is what will most likely get you in the door.