There's different Picturviewer programs belonging to different os's & brands. So it's a bit difficult to trace your problem. By sounds of it, you can't open the file b/c your image viewer doesn't recognise that format. ie: theirs different variations of each format. Even jpg can have different variations. It's hard to say. Did you try opening it from within LW's library window? If yes, try opening it outside of LW.

Oh if it's happening with non-image files then perhaps you should set your defaults for each file-type. I also hope you haven't been a victim of autogenerated spam results