How to set read-only dmg background picture in Mountain Lion? Until MacOS 10.7, I used to put a png file in a read/write dmg, then I set it as its background picture, then I hided it and converted the dmg to a read-only one. Everything just worked.

Now, in MacOS 10.8, if I do the exact same thing, when I open the read-only dmg it opens the read/write one too. It looks like it "knows" the original background picture is on the read/write dmg. Even if the read-only one has the same image in its contents.

How can this be prevented?
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.