Hi,
The plugin is using the library "html2pdf" which transform an HTML page into a PDF file.
So if you want to put a background image, the best will be to edit the HTML to put that background image.
You have to know that "html2pdf" understand the CSS "background-image" rule, you just put an image url that your server will be able to understand/read/access.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.