Hello, the background image can be placed under Style properties -> Notices -> Welcome notice. You need to set "Welcome notice background" to None, at which point you can use the Welcome notice style property at the bottom of the page to upload your own image / change the color.
Adding an image, you'd just need to add it to the template "xb_welcome" and insert it where you'd like it to be placed. You may need custom CSS for it to work.