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.
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.
http://the.beag.co/