Currently, the Sugar default PDF templates contain a logo file embedded in the template for SugarCRM. However, there is no clear mechanism to embed another image unless the image is hosted at a URL outside of the SugarCRM instance. Per the How can I change the SugarCRM image on the PDF quote template? discussion, it is possible to upload additional images to utilize in PDF manager, but it is not an apparent solution to admin users.
I think we should expose this functionality in a direct fashion (e.g. upload tool and image selector option in the toolbar) for admin users to manage images they wish to embed in PDF templates rather than relying on static URLs outside the Sugar instance.