SO! (…of my understanding) ~ They’re paintings within other work of art executed directly on a wall, ceiling or a vast permanent exterior. A distinguishing quality and its’ characteristics of a wall painted canvas is that the architectural elements of the known space are pleasantly built into the portrait. This technique has been in common use since the late 19th Century to this end product effect of either a hand-made painting or a realistic scene. A scenery of Love, Hatred & Loss. ~ It’s by finding the location: This is the first and the most important one of making a mural as it should be flat and smooth surface, but planning this can be difficult, so rough textured surfaces may work as well.
Here ends, my theory on Murals. Short, Brief as I kept it small as can be. ;P
P.S: Here’s my example of how I made mine. (see BELOW)