To add a background image, simply select "Add Background" on the banner which is visible in the upper left corner of your invoices and the upper right corner of proposals.
If you do not see the option to add background or edit, it may mean that you're not in 'editing' mode. Documents that were already sent or being viewed in client mode cannot be edited so you will not be able to add a background in these circumstances.
Images being added as a background or logo should be no larger than 5MB and be consistent with the following dimensions:
Proposals Background Dimensions:
858px x 200px (non-retina)
1716px x 200px (retina)
Invoice Background Dimensions:
658px x 150px (non-retina)
1316px x 300px (retina)