You need to have really decent pictures to earn money on stock sites. My friend sells them on one of the stock sites and he is doing very well (he is doing it as full time job), but he pays models, for studio hire or special locations (dentists, opticians, car mechanics, offices etc.), furniture, equipment and other stuff. His main subject are photos of professional people. Often budget of £1000 per session is not enough if involves specialist equipment and 4-5 models - it's a lot of money, so you need to have proper material to sell it well and many times. Other thing are criteria that you need to comply with - top image quality, usually ISO max 200, stay away from registered trademarks (even little logo of manufacturer on equipment will disqualify your pictures) and you need to get through "judges" who will (or not) approve your images.