If you have a document or an image that you would like to attach to your document templates (budgets, proposals, invoices), you can do that by going to Settings - Document templates, choosing the template you wish to edit (or create a new one), and adding it under the Customization - Attachments section:

As indicated in the information popup, you can upload PDFs as documents, and images in JPEG / JPG / PNG / SVG. GIFs are also supported!