Basically you need to look at two things:
- What you are hanging (how heavy, will it sway, is it a vertical load or does it have a horizontal component)
- What are you hanging it on (wallboard, plaster, old crappy plaster, stud wall, concrete wall, brick, stone, solid wood...)
Then you pick the right fastener at the intersection of these two.
Assuming you have a newer house, and are mostly hanging pictures, the old "nail into wallboard" will work okay. Just tilt it downward so the picture wire slides toward the wall, not toward the nailhead.
You can also use things like the 3M "command" strips, which work great if you follow the instructions and don't exceed the load. Don't be stingy with them.
Hanging heavier things like mirrors or glass picture frames, use the appropriate wall anchor for the wall, and/or nail or screw into the studs. With the right wall anchors, or rails or other structure attached to the studs, you can put an enormous load on the wall (tons) without problems.
If you have an old house like mine, don't even bother trying wall anchors into ancient crumbly plaster...find the studs. Hard-won lesson. ;)
Using a stud finder can be a bit of an art, so practice on a wall you don't care about. (Mark the edges of the stud, then drive a very fine nail into the wall board and see if you were right. Then, pull the nail and spackle the hole.) Also note that they read differently depending on what kind of wall it is.
Accessing any substrate as jamietre suggests is probably the best bet if possible. Otherwise Tester101's epoxy will be best. The sort of shelf you want places fasteners in withdrawl, which is a difficult load to resist in marginal materials such as bungaroosh. If you attempt an epoxied anchor, you should attempt to undercut the hole with a small hard chisel to further help resist withdrawl forces. This way, you are not entirely dependent on the epoxy bond strength. Unfortunately, you still need to depend on the shear strength of bungaroosh anyway, which is marginal at best.
The deeper the hole, and the greater the undercut without damaging the surrounding wall material will yield the strongest anchorage.
Best Answer
Your wall looks like plaster. It is most likely laid over wood lath, metal mesh or backing board.
Sometimes you can bang very thin sharp nails into plaster successfully. But yours seems to be more brittle by your description.
Try drilling through the plaster with a small sharp drill bit in a few places. Feel when the plaster transitions to something else. Then clean the drill bit and keep drilling, looking at the shavings to determine what your backing is. If it's wood lath you may happen to drill between laths, that's why I say "in a few places". You could also drill a half-inch hole and buy a cheap endoscope to see definitively what's behind the wall. (Edit: Some say you should use a masonry bit. I get better results with a sharp wood bit but you should experiment to find what's best on your walls. And for the initial experimentation I describe above, the ideal thing is a hand drill. And there is an additional possibility I should describe: the plaster may be over brick in some places, eg over a chimney. In that case you should use appropriate methods for brick.)
If it's metal or backing board you can use plastic anchors for plaster (don't use drywall anchors) or toggles depending on what you're hanging. If it's wood lath my preferred technique is to drill just up to the wood, then drill a small pilot hole through the lath, put a shortened plastic anchor into the hole (it will only go up to the lath) and then use a 2-1/2" screw. The anchor will do some of the work and the lath will help a great deal. The combination is quite strong.
I also second the advice in the other answer about using the picture rail as intended. If your picture rail is an ACTUAL picture rail (it's hard to tell from the photo) it will have a rounded top designed for special picture rail hooks. Buy them. Then don't need ANY tools or fasteners at all!