What I normally do is use the “background image” field for those sorts of images, as it isn’t constrained and I rarely use actual background images.
Functionally, it’s a totally standard signup page. Everything — even the background image — is editable via the Control Panel, and it can be reused as many times as need be.
If you have, and you’re still having difficulty, please consider contacting one of our certified NationBuilder Architects: http://nationbuilder.com/architects
If you’re going to go that route, I’d strongly recommend pairing Mailboxes with a third-party service, as NationBuilder does not enable mobile email (on your mobile device), offer spam filtering, etc. With a Google Apps (or other provider) + Mailboxes pairing, you’ll get the best of everything.