1. From Customize section, go to Background Image.
2. In Background Image Options you will see:

• Background Image: Select the image for Background available on media library or upload from the other location.
  If the preview is fine, click on Save and Publish to apply recently selected/uploaded image as Background Image.
• Image Position :You can adjust the image position according to the arrow.
• Image Size : You will be able to choose various options from the dropdown. (Fit to screen , original , Fill Screen) you can choose any as your requirement .
• Repeat Background Image: You can check this to repeat the background image .
• Scroll with Page: You can check this option to scroll the image with the page .