Thats coming from your background image uploader. That usually happens if you’ve ever added an image, or attempted to add an image in the background image uploader. All you would need to do is go to Appearance > Customize and then click the “Background Images”. From there you can go to each section you’re not using and click the remove image option.
Some people use the background uploader if they don’t know css….while some would rather add the background image to their css directly….which means you wouldn’t need to use the uploader.