Hello,
Out of the box BILLmanager contains a number of pre-defined document templates. You can edit them or create custom templates according to your needs.
Navigate to Document templates -- > and use contents of the document template in XSLT.
If you want to upload a logo or other image, you can do it into usr/local/mgr5/skins/userdata.
They will be accessible at
https://billip:1500/manimg/