I'm assuming that your running in os9? Given the Forum, I hope that's the case.
If you're in os9, then read on, if not, let me know and we'll think of something else.
If you have a .img file, then you should consider using the following techniques to open it.
Disk Copy will mount image files, but sometimes I've found it doesn't work well with .img files that were compressed using DropStuff.
Try using Stuffit Expander, make sure you have a new version of the utility... and make sure that you're in os9 when you do it.
Again, maybe you've already tried this, and if so, I'm sorry for the distraction... but it works for me 95% of the time when DiskCopy fails.
The other option is hand me the file and I'll crack into it some way or another.