In order to produce the final visual style for your site, elements from Customizable Elements are combined with elements from the Custom CSS tab of your site. Some designs on Squarespace rely on items in Custom CSS to specify advanced effects related to positioning or images that might appear on your site. To customize or remove these elements, remove lines that are appropriate from Custom CSS.
Which lines are appropriate? If you're trying to remove a background image that doesn't seem to be specified in Customizable Elements, there are most likely only a limited number of backgrounds specified from Custom CSS. Try removing them. Feel free to ask us for help doing this if you run into any problems.