DMG disk image error: no mountable file systems

My Macintosh HD recently got bad sectors and it wasn't booting. I tried Disk Utility repair, but it failed. So I cloned the entire disk and did a clean installation of Lion. I had a disk image of 30GB in my old Macintosh HD in which I used to save my data. But now after the clean installation, when I tried accessing the disk image from the backup, it gives me an error - "The following disk images could not be opened" and "no mountable file systems".

I really need something that can fix this and give me access to my backup data. Thanks in advance.