Hanging Large Framed Prints on Plaster
The best bet is to find a stud and use a heavy nail or screw. 

If that isn't an option for the print's sake, I've had luck in such walls with toggle bolts before. It can be hit or miss with screws and even mollies and screws in tandem because behind the plaster and cement are 3/8" thick by 1 3/8" wide strips of wood spaced 3/8" apart called lath, which is what the gray cement (the "scratch" coat) squishes into when wet and locks into once dry. You'll drill a hole and you might hit wood (lath), or you might hit air, and which you hit will affect a fasteners ability to hold in the hole, and it's often a crap shoot on the first hole or two going in blind. A toggle bolt works well in these walls because the "wings" of the bolt have a good chance of catching the back of at least one strip of lath and hopefully more, no matter where a hole is drilled, giving it a good purchase and a high chance of success on the first try. 

Don't even bother with nails in these walls unless your certain you're driving one into a stud. You're liable to break loose burgs of cement and plaster, and yes, bend nails, because the elasticity and memory of the wall across open space is high and the energy of your hammer blow will be mostly directed back at you and the nail itself.

You'll want to use masonry drill bits for any drilling in these walls. The cement beneath the skim coat of plaster will chew up any others almost immediately.