line = 4;
mes = new Array(4);
mes[0] ='<img src="img/c1.jpg">';
mes[1] ='<img src="img/c2.jpg">';
mes[2] ='<img src="img/c3.jpg">';
mes[3] ='<img src="img/c4.jpg">';
Rnd = Math.floor(Math.random()*line);
document.write(mes[Rnd]);