I'm going to be a bit harsh and say that your cause is doomed to fail if you want to solve the problem just by policing students.
What happened is that some factors contributed to atmosphere where it is considered advantageous to have a copy of past exams and your students are motivated enough to risk getting a copy. The tradition of having a monetary incentive makes things even worse.
At my previous university, I had opportunity to witness the evolution of hi-tech copying systems. At first, nobody was checking for cell-phones because they had bad cameras and weren't as popular, then cell-phone copying became popular. Then cell-phone detectors of various levels of sophistication came into use. After that, cheaters moved on to other devices.
Today, spy devices are cheap and commonly available and they are next logical step from cell-phone cameras. Are you going to start checking your student's watches next? How about buttons, glasses or even pens? What about say calculators (OK, that one in particular isn't the best example, but if there's demand, supply will come), whose use might even be allowed in some examinations? Are you going to have a spy-equipment expert on your staff to check what your students are using? What if they home-brew some equipment?
What if someone actually steals a physical copy of questions? That actually happened at my previous school. Guy (not a student at the school) came to a lecture hall where an examination was being conducted, waited for TAs to hand out the questions and then proceeded towards the exit with a question sheet, running over anyone who tried to stop him. There was even a recording from video-surveillance of him doing the deed and a wanted poster was placed at the school entrance, but it didn't do any good. That particular problem was solved by asking for IDs before handing out questions, but it shows the trend of escalation that can happen.
Next, what if a group of students organizes with the idea of memorizing questions in detail without any technological aids? There's literally no way of preventing that.
The more you press the anti-technological/anti-cheating offensive without taking away the incentive to cheat, the greater is the risk that you'll instead form a core of semi-professional cheaters who will have connections to the sources of appropriate cheating equipment and serve as a cadre which will train future generations and make problem even worse. For example, in my hometown, a sure way to detect presence of a higher education institution is the high concentration of flyers advertising rent and sales of spy equipment.
The only sure way (that I at least can think of) to solve the problem is to cut it at its source and take away the incentive to have the pictures of past exams. Try to take time to analyze all factors that could lead to such behavior and see if you can actually affect any of them in a meaningful way. Although questions aren't repeated, it's obvious from the response of students that seeing past exams is beneficial in some way. It's normal for questions to be similar, since there are probably some underlying concepts that students should learn and that knowledge needs to be tested. If the students are already aware of what they're going to find at the exam, then there isn't much need to see how exactly the sheet with questions looks like. If the exams is supposed to be a surprise, then you should reconsider if you're actually preparing your students properly for the exam.
If going in-depth when solving a problem such as this isn't real motive, then it would be best to take advice from Moriarty's answer. You'll be doing something "direct" and you probably won't challenge existing policies too much.