In 17.3, the background image (once obtained) is apparently parked in the <office:binary-data> after any filtering.

All that 10.4.5 <office:binary-data> provides base64binary of the image, but we are given no clue what the represented image format is.

There must be more information in either <office:binary-data> or, if not that because the element has multiple uses, in <style:background-image> for a consumer to be able to render the represented image or to determine that it cannot.