tips = new Array(2);

tips[0] = '<a href="" target="_blank"><img src="/upload/201791013391528960.jpg" border="0"  width="250" align="left" alt="gg"/></a>';

tips[1] = '<a href="" target="_blank"><img src="/upload/201791013385814874.jpg" border="0"  width="250" align="left" alt="ad"/></a>';

index = Math.floor(Math.random() * tips.length);
document.write(tips[index]);