var imgMax  = 2; //セットする画像の枚数。
var imgRand = Math.floor(Math.random() * imgMax);

if(imgRand == 0)       document.write('<IMG src="ctr_img/ctr_rdm/ctr_plf_gso/gso_pht_1791.jpg" width=300 height=400 border=0>');

else if(imgRand == 1)  document.write('<IMG src="ctr_img/ctr_rdm/ctr_plf_gso/gso_pht_0842.jpg" width=300 height=400 border=0>');