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?