Hey Jayd,
Hope you’re well today.
There is currently no built-in option to add a background to the canvas menu. However, it is possible by adding some custom CSS to the element through the element’s Element CSS feature.
Please follow these steps:
#1. Edit the navigation element option then click on its Customize option.

#2. Add this code to the Element CSS field:
$el.x-off-canvas .x-off-canvas-content {
background: #fff url(URL_OF_THE_IMAGE_FILE) no-repeat center center;
background-size: cover;
}

Please do not forget to change URL_OF_THE_IMAGE_FILE
with the image file URL in the code above.
Kindly note that since this is a custom code that changes the default behavior/display of the theme, you will be responsible to maintain or update the code in case you require further changes or if the code stops working in future updates.
I have also added this feature in our Feature Request list so that it will be considered in future development.
Hope this helps.