You first must create a "Game" object in Phaser.

var game = new Phaser.Game(800, 600, Phaser.AUTO, 'phaser-example', { preload: preload, create: create });

In the preload callback function load the image.

function preload() {

    game.load.image('thing', 'assets/thing-image.png');

}

Parameter	Details (Game.add.image)
name	the name used to reference the image in the game.add.image method.
file	path to the asset file (relative to the root directory for the project.

Then in the create function use the "add" method of the game object to create the Image object and it to the screen.

function create() {

  var image = game.add.image(100, 100, 'thing');

}

Parameter	Details (Game.add.image)
x	the x coordinates where the image should be added.
y	the y coordinate where the image should be added.
name	the name of the image assigned in the game.load.image method.