How do I paint art onto a wall?
You are painting a mural, like Leonardo or Michelangelo.

It is a little easier now since the invention of acrylic artist paints. Modern acrylic/latex wall paint shares many characteristics, but generally is not as rich in color and does not have the durability to resist fading as well.

If you want the best finish and durability, you should coat the wall with a special art primer called gesso. This is a base that holds the finish paint well and is very flexible to resist cracking and other breakdown. 

The colors for the design should be artist acrylic paints. There are student level paints that are cheaper than professional grade paints for covering larger areas. If you need to stretch the paint, you can add mediums, but his will give a slightly less intense color. If you want to paint a background color using regular wall paint, you can use a clear gesso. This will save money over having to paint the background with artists acrylics, but the color will probably be slightly less stable than artist colors.

The finished work can be coated with an acrylic varnish, either brushed or sprayed on. This will protect it, but it should not be scrubbed the way you might a conventional flat painted wall.

Information about these materials can be found on the sites of various acrylic artist paint manufacturers.