How to change the background of an DMG Software License Agreement dialog?
One easy way to create DMGs and play with every option is to use the popular DropDMG from Michael Tsai. It's not free, but he has great support and it's very communicative and there's a trial version.

Disclaimer: I'm not affiliated with Michael Tsai, but I've known him (and his products) for a long time now, when I started using SpamSieve (another of his great products).