var imageArray = new Array(
'<img src="images/bird.gif" width="80" height="70" border="0">',
'<img src="images/squirrel.gif" width="80" height="70" border="0">'
);

var imageIndex = Math.round(Math.random()*(imageArray.length-1));
var currentImage = imageArray[imageIndex];

document.writeln('<div id="animal">');
document.writeln(currentImage);
document.writeln('</div>');