I have had that problem with .img files. I can not place when it began.

I open them in File buddy and reset the creator type to my version of ShrinkWrap 3.0.

I have other image files that won't work with that trick. Maybe a newer version of Disk image mounter. I would guess it is a 9/10 thing and not a LW thing.... not too sure. it's hard to part with a 200mb file that won't mount after all the time it takes to get it.

Another weird thing, I attempted to upgrade to ShrinkWrap 3.5 and the installer said I did not have the correct version(3.0) installed?? time to start over.. ;P

what's the .dmg app called?

I'll post if I get a resolution to the mounting thing...