You can't duplicate a movieclip which is in another movieclip, only movieclips that are on the same timeline as where you run the code from, and besides, anything inside the brackets will be recognized as ONE object, not two, even if you have a dot, because it's a string, and all codes will be ignored in there. To give you an idea of how you could've done it if your movieclip was duplicated:
ActionScript Code:
_root.mc["mcImg"+i].loadMovie("image.jpg", 50);
btw, why not just execute the above code inside the movieclip,
mc, and instead use this duplicate code:
ActionScript Code:
duplicateMovieClip (mcImg, "mcImg" + i, i);
?