var mess = new Array(

'<img src="http://www.funny-jokes-and-pictures.com/images/Animal-Funny-Pig-Happy-Grin-Picture.jpg" class="left" />',

'<img src="http://www.funny-jokes-and-pictures.com/images/Animal-Funny-Horse-Laugh-Picture.jpg" class="left" />',

'<img src="http://www.funny-jokes-and-pictures.com/images/Animal-Funny-Cow-Tongue-Laugh-Picture.jpg" class="left" />'

);

var max = mess.length;
var num = Math.floor((Math.random() * max));
document.writeln(mess[num]);