There's a world of difference to how artwork looks when hung by an expert
compared to
when it's hung by somebody who lacks knowledge, skills and all the kit relevant to hanging pictures

TIPS: How to hang paintings, drawings, fine art prints and photographs.
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Below are some basic tips for hang your pictures - whether they are paintings, drawings, prints or an object d'art.

Structure of the wall
  • always know what's behind the wall you're going to be hanging on. If you do't get yourself a tool to check for wires or pipes
Arrangement
  • try arranging the group of pictures on the table or floor so you find a pleasing arrangement BEFORE you start to hang
  • think about the bigger picture created by the individual pictures look and apply the same rules of composition and design that you might you use when making a painting
  • decide whether you are going to group around a (1) centre line (horizontal or vertical depending on the nature of the space for hanging) or are building up from (2) a bottom line (for horizontal hangs) or a vertical (eg x" from the vertical of a door frame).
  • don't hang pictures so people have to strain to see them
Getting everything level!
  • use a spirit level to make sure the horizontals and verticals are correct  (I keep a spirit level app on my iPhone)
Safety first
  • assess the weight of your artwork including frame BEFORE choosing hanging fixtures for the picture and the wall. You need to be very sure it isn't going to fall off the wall!
  • a two point hang for larger pictures is much safer and more stable. 
How to hang art in your home
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  • Keep it simple: choose your frames and mounts with care and don’t clutter up the wall space
  • Hang pieces at eye level. Most galleries hang their art roughly in the middle of the wall, so this is where the middle of your piece of art should be, not the top
  • Use picture hooks, rather than nails or screws, so as not to ruin the wall
  • Double up on picture hooks rather than using a wire so you can properly position your piece of art. That way, it’s stationary on the wall and not swinging on a wire
  • Finally, get out a spirit level and, if you like, a ruler, to make sure that you have everything straight.

  • Why you shouldn't be scared to hang art in your home | The Telegraph

Different ways to hang a picture with a frame
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BASIC KIT for hanging a picture
  • ​Hanging hardware for the picture (D rings)
  • picture wire or picture cord
  • Hanging hardware for the wall (e.g. Hooks with steel pins OR mirror plates OR heavy duty plates )
  • picture rails (optional)
  • spirit level
  • pencil
  • measuring tape
  • a small hammer
  • a Phillips screwdriver
  • an electric drill (for solid walls)
  • appropriate screws and associated rawlplugs ​
D ring screwed into a wooden picture frame with picture cord correctly tied and whipped
D ring screwed into a wooden picture frame with picture cord correctly tied and whipped

D rings

D rings come in different sizes, different metals and can carry different weights. Be sure to check what sort of weight or type of picture they are suitable for before buying/screwing them into your picture. If you're hanging a heavy picture you really need to move on to a different type of hanging hardware.
Tips for using specific frame hanging fixture kits
Picture
A small artwork framed using D rings as hanging hardware. Note how (1) the professional framer has used single strand of picture cord and then knotted and and whipped the ends (2) the back of the frame uses brown tape to seal the backboard to the frame.

Hanging wire and picture cord

The UK and USA vary in their preferences for heavy duty picture hanging wire - the UK likes brass and the USA likes steel. 
Whatever it's made of the really important thing is to check the load it will carry. Plus choose a wire appropriate to the load.

Below the brass wire on the left will carry 50lb and the vinyl coated steel wire on the right will carry 43lb (20kg). Plus don't forget that lbs are not the same as kg!

In the UK heavy duty picture cord is also very popular - and a lot easier to tie off than wire.

DO NOT USE STRING - string is not designed for hanging and stretches and frays. It's the fastest way to have a picture fall off a wall

Whatever you use it needs to be "whipped" at the ends to ensure they don't untie or unravel.
Below is a video which provides instructions on How to Properly Knot Picture Frame Wire. The same technique applies to picture cord except you need to fix it so it will hold (see pic above)

How to hang and align artwork
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Below are links to general articles about how to hang a picture and get it correctly aligned - vertically and horizontally