It's easier to buy a picture than to hang one. Here are some tips from Eoin Lyons
A good framer should provide fixtures to go with the frame, and cord or wire strong enough to match the picture's weight. For most purposes, a standard picture hook driven into the wall provides enough support, but particularly hard walls will require hooks with three or four pins specially made for this purpose, available at hardware stores. Where the wall is plaster, it may be advisable to drill a hole, using Rawlplugs and screws.
Use two hooks spaced apart if the picture is heavy or particularly long. Anything very heavy - large oil paintings in old gilt frames or big mirrors - can be hung using two ropes of picture chain, suspended from two screws in the wall above, so the weight is spread evenly. If this sounds a bit old fashioned, you could attach brass wall plates and screw the painting to the wall.
Consider light levels. Keep watercolours away from walls that get direct sunlight. Don't hang anything above an uncovered radiator. Where possible, have a shelf fitted above the radiator to deflect the heat.
Stand your pictures around the room and shuffle them about to come up with different combinations. Consider how paintings look against other paintings, how they appear in relation to fabric colours and furniture and how they fit into the wall space available.
If you want to hang a group of paintings, lay them out on the floor and move them around to find a good balance. If they differ greatly in size, use the biggest as your starting point. A large painting has a big influence on how the room is going to look and there are probably only a few places it can hang.
It might help to create imaginary lines on the wall - a horizontal line with pictures arranged above and below, or a few vertical lines that make centre points for column arrangements. Keep the same amount of space between each picture frame. Juxtapose horizontal and vertical paintings in a small group.
Hanging a set of pictures that are the same size - perhaps prints by one artist - in a straightforward way is often the most logical way to arrange them because the intention is usually that they be viewed as a whole. Squaring them off in a block provides a focus to one area, but requires careful measurement to ensure they all stay in line.
Another approach to picture hanging is driven more by interest in the pictures themselves than by achieving a decorative effect. This creates a look that reflects the natural growth of a collection. This tends to involve a mass display, so keep bold, large images high and more detailed ones lower, so they can be seen easily. This often works best on a staircase.
The walls and their colour set the mood of a room. Dark colours make traditional oils and works without mounts stand out. Pale walls are better for watercolours and modern works. Fabric covered walls are more likely to suit delicate pictures. Boldly patterned wallpaper calls for strong images in strong frames.