To add a background image to any proposal template or invoice template, simply select "Add Background", which is visible in the upper right corner of your proposals and the upper left corner of invoices.

If you don't see it, make sure you are still editing your proposal/invoice.  If it's already been sent, or you're viewing the client version, you can't edit the background image.

Proposals Background Dimensions:

858px x 200px (non-retina)
1716px  x 200px (retina)

Invoice Background Dimensions:

658px x 150px (non-retina)
1316px  x 300px (retina)