Background images are just that, images that go in the background. Don’t use images with anything too important in them. Just choose something to lift the mood, such as a body of water, a forest, or the like.
Any images with logos or text shouldn’t be used as background images, because background images don’t scale automatically and are often cropped by the devices event invitations are read on.
If you have an important image, you should instead use a regular image block.
If you want to delete a background image, click on “Use background color” instead.
A number of background images are included in the system. In the image selector, click on “Image archive” and search for “bg-”.