I'm Daryoush, on the superb Guru team! :)
The code customization required to add a custom background image to the website footer is different for each theme. It is usually done by adding the background image in the "Assets" folder in "Online Store" -> three dots -> "Edit HTML/CSS" then using this image in the theme.liquid or footer.liquid by using a call variable. If you are using a Shopify supported theme, you have access to 60 minutes of free design time, please contact us and our Theme Support team may be able to assist! In the meantime, perhaps an awesome developer or Shopify Expert can help out in here!
I hope this helps! Please feel free to give us a call or start a live chat at any time, we are open 24/7 for your convenience and always happy to help!