Sometimes it's the simplest jobs that can be the most difficult. I'm willing to bet that its the summation of tiny errors leading to a visible flaw at the end. The question you want to ask is "How do I mark and drill a hole exactly where I want it to be?" (For a given value of exactly)
The answer to this question (and many others) is: Make a jig or paper template. Even if you're only doing it once, making a jig or a template is essential for precise work.
For this particular job, a paper template will probably do. Trace the outline of your hook (and holes) onto a sheet of paper. Now, at your leisure, using compass, straight-edge, GPS, and Lasers, find the center of the holes. Even if you just eyeball it, you're still more accurate than doing it on the wall, as you're working at a desk or table with good lighting and a comfortable working position.
Now, photocopy that sucker!
Cut it out, tape it to the wall, and start the drill hole with a small nail or even punch it with a nail set. You don't want the drill "walking" away from its start position. Go in with a small bit next. The idea is that the tool will do all the work so that you can concentrate on keeping that drill level, and on center. Go to your 1/4 inch bit for the plugs, and clean out the hole.
Remove paper, insert plugs, and you're golden.
It may seem like a lot of work, but the results are worth it.
1) Generally, you can't be accurate working vertically, so figure out a way to do the detail work on a flat surface.
2) Always start a drill hole with a nail or punch. It will always walk away when you power on the drill. (This is quadruply true on metal or hard plastics.)
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.
Best Answer
There are extra sharp, thin nails used in several brands of picture hangers that reduce the chance of cracking or chipping plaster They also can be used alone if the item is not too heavy.
You also could use an anchor and a flat head screw and then merely glue a decorative button or other item from a craft store to the head.