var ar = new Array("<IMG SRC=html/image_inside1.jpg width=171 height=166 border=0>", 
"<IMG SRC=html/image_inside2.jpg width=171 height=166 border=0>",
"<IMG SRC=html/image_inside3.jpg width=171 height=166 border=0>",
"<IMG SRC=html/image_inside4.jpg width=171 height=166 border=0>",
"<IMG SRC=html/image_inside5.jpg width=171 height=166 border=0>",
"<IMG SRC=html/image_inside6.jpg width=171 height=166 border=0>"); 

var num = Math.floor(Math.random() * ar.length);
document.write(ar[num]);