They are usually a photo set associated to a movie, but not necessarily taken from an exact scene.
http://www.wicked-pornstars.com/galleries/apr2011/jessica_drake_lesbian_dolls/index28.html
When Michelle B was coming to the US to shoot scenes, she was doing 2 or 3 scenes a day for a week or two straight. Reading ChristianXXX's blog it wasn't unusual for him to mention the gal was just coming from another scene or had one booked for later.