You didn’t specify what version of Manager you have. Current version is v16.12.48, and view templates for sales invoices have been obsolete since v16.11.1. Themes are now used instead. Regardless of your version, update now. Custom themes were added at v16.11.24 and are explained here:
Inclusion of a graphic image in a theme, especially if limited only to sales invoices, is beyond the scope of this forum. But an HTML programmer with some knowledge of Liquid templating could do it.
Actually, I think you could put an image in the Notes field and suppress the field title with a filter. But that’s beyond my capability.