var banner = new Array();

banner[0]=https://i.imgur.com/Njp6u.png;
banner[1]=https://i.imgur.com/RDVfV.png;
banner[2]=https://i.imgur.com/o98J6.png;
banner[3]=https://i.imgur.com/zkQ0H.png;
banner[4]=https://i.imgur.com/KAZ0v.png;

document.getElementById('logo').firstChild.src= banner[Math.floor(Math.random()*banner.length)];