How to set read-only dmg background picture in Mountain Lion?
You're right. The original disk image is opened when opening a newly converted read-only image.  However, I found that if you delete the original read-write disk image, Finder (actually, DiskImageMounter) has no problem mounting the read-only image without its original.  DiskImageMounter itself is not really responsible for the behavior you are talking about; if you look at your read-only image's .DS_Store file in TextEdit, you'll see (amidst very obscure symbols) your original image's name and file path in many places.  This is just built-in behavior of Mountain Lion, I suppose.  Unfortunately, there are no workarounds except for deleting the original image (and emptying the Trash) because Disk Utility is ultimately responsible for the creation of the DS_Store file, and the image created is read-only.