When I downloaded Skype, I noticed that, in place of a normal \"disk image\" icon, there was a custom icon, and when I double clicked on the icon, the window used a colorful ima
Let me add to the other answers jwz's howto on setting view options on .dmg files. This is the simplest one that I got to work with no problems.
Also, to change the icon from the default .dmg icon:
In the Finder, open an inspector window (^I) for the file whose icon you want to use, click on the icon at the top (it will get a blue border) and copy it to the clipboard (^C).
Then open another inspector for your .dmg, click on the icon and press ^V to paste
(you can change the icon for any file like this).
For this kind of thing, you can always look at open-source programs and see what they do to get a similar result. For example here are the relevant source files for building pretty dmg files in: