If you want to distribute software then you may want to automate the process (it gets very boring, very quickly, doing it by hand.)
Might I suggest that you investigate the process that Growl uses. You can even integrate it into an xcode build phase.
If you want to be really lazy (like me) and don't need xcode integration you can use a small AppleScript Droplet I made (after examining the Growl build process a few years ago.) It needs to be an applet bundle because it contains the 'openUp' executable inside it. (i.e. the script alone will not work.)
I forget exactly how it works, but you should be able to work it out by looking at the source of the script.
As far as I remember you just drop a folder (laid out as you wish) on to the droplet containing an image named background.png/jpg at the root of the folder and it will create a DMG for you....
http://www.diggory.net/grazing/DMGBackgrounder.zip
good luck.
p.s. more info on growl build process from the blog of the project leader here:
Nobody learns if nothing is brok3n: September 2006 Archives