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.
Given that it's an exterior wall you've probably hit brick.
This would explain both why you've not found a stud - there aren't any, and why the nail only goes in 1/2" - 1" - you've hit the bricks.
Given that the plaster is crumbling, the simplest solution would be to drill a hole with a masonry drill and use a screw (or screw hook).
If you want to hang a lot of things on this wall then it might be a better idea to install some sort of rail - either a traditional picture rail or a more modern alternative and hang the pictures etc. off that.
Best Answer
If the frames aren't important & are at least a few mm's wide, then it would be best to actually secure the pictures by drilling the frame & screwing them directly to the wall. Either, directly into the structural framing or into one of many available wall anchors, I prefer one's that flare-out behind the plasterboard.
You can also accomplish the same result by using large washers on screws applied to the outside of the frames (2 on bottom & 1 on each side), so there's no drilling. But, the washers can be a bit ugly. So, the better option would be a Mirror Hanger type of Wrap From Behind clip (left) or Perimeter Offset Clamp (middle) or you may have to bend & possibly cut your own out of L-Brackets or Corner Braces (right) to accommodate thicker frames (Wrap From Behind application).
These will handle a very good deal of weight & frequent bumping into & can be padded with tape or felt to protect the frames.