Originally Posted by patryk
you would need server side code for that.
That is very true, but depending on how many images you have you could go some way to achieve this by showing an image based on the time (in seconds) that the user enters the site (or clicks the image to change it). Obviously that does not work if two or more users enter the site or click the image at exactly the same moment. It will also be unreliable if you only have four (or a small number of) images.