There is an option for changing background image in Customizer
Here are the steps you need to follow
Step 1) Login to your WordPress Admin Panel
Step 2) Go to Appearance -> Customize
Step 3) Click on General Settings tab
Step 4) You will see Home Background Image. You can upload your image from here.
Step 5) Make sure you click Save & Publish button at the top to save changes
Please do not hesitate to post your questions in our support forums. Our support team will always there to help you