Correct, which is why I did the following:

////////////////////////////////
string s = "\\\\myserver\\data\\images\\myimage.jpg"; //will not find this

string lc_file1 = Server.MapPath(s) //Now it should find it

string lc_file2 = "c:\\media\\images\\myimage.jpg"; //this will be found

Give us some more information on the systems you're attempting this for...