Fix PHPOffice#1649, a 3-year-old issue long marked "stale". Excel supports background images on sheets; now PhpSpreadsheet will as well. Support is limited to Xlsx (read and write) and Html (write only). As far as I can tell, Excel Xml and Gnumeric do not support this, nor, of course, do Csv and Slk; Excel Xls does, but, as usual, how to handle it in BIFF format is a mystery; LibreOffice ODS supports it differently than Excel, and this is just another of many ODS style properties not currently supported by PhpSpreadsheet.