open an .img file - "Not Recognized"

I have some files that were made (I assume, a while ago) into a .img file. When I double click them, some mount and copy just fine, but others will pop up a Warning window that says:

The Following disk images failed to mount
Under the Image menu, it gives the name
Under the Reason menu, it says "Not Recognized"

Is there a program, or something I can do to pull the info out of these .img files?

Thanks!
Mike