DropDMG 2.8.5’s Use custom icon for mounted image feature does not always work property with Mac OS X 10.6:
  1. If there’s no icon file in the source folder, DropDMG will generate an icon that badges the application icon onto the white “mounted disk image” icon. When running Mac OS X 10.6, the icon may be incorrectly scaled.
  2. If there is an icon file in the source folder, DropDMG will correctly apply it to the mounted disk image.

To work around the problem in (1), you can either turn off the custom icon preference or make sure that the source folder contains a pre-generated icon (2). To save an icon file in the source folder, you can:

When a fix for this problem is available, I will update this post.