for (ycell=1;ycell<=20;ycell++)
	{
	document.write("<tr>");
    for (xcell=1;xcell<=20;xcell++)
		{
		document.write('<td><img SRC="gris.gif" height="24" width="24" onMouseDown="situa('+xcell+','+ycell+')" ></td>');		
		}
	document.write("</tr>");
	}