To upload an image from the Background button (located in the tool bar):
From within your calendar, select the Background button in the tool bar. Click on Upload button in the Image Library pop-up window, and then drag your image or click to choose an image from your computer.
To upload and image from the Image icon (located in the tool bar):
From within your calendar, select the Image icon in the tool bar. From there you will see a Choose... button under Insert Image in the pop-up window.