Is this not just extension interpretation? eg .sit= stuffit file for mac; zip= zipped file for pc ... media files .jpg, .mpg, .txt, .doc seem to work equally well on either; I personally have problems with .asf and .avi on my mac but I think this may just be my problem; .dmg definitely work on this mac, I speculate that .img is windows version but I don't know and would be glad to hear- actually, now I think about it, since macs can read pc format disks they should cope with pc format images, shouldn't they?

As for not opening: check info for the file. If it says type LimeWire Document, you will need to drag the file on to the application that deals with it; Stuffit expander, as the man above says, for .sit (and .zip, I guess, though unless they are zipped media files you are wasting your time, I think); and diskcopy , Mac HD/Applications/Uitilities for .dmg (and possibly .img).

Apologies if you've tried all this: at least you're not as dumb as me- the first time I unstuffed a file to a .dmg I thought this meant 'damaged' (I'd never heard of disk images, or diskcopy) and threw it in the trash- all 50 megs!!! Please shoot me...