As a keen photographer, I have hundreds of my photographs on my computer, so I can then change them as often as I like.
We have just had a new kitten this month so it's his turn.
http//farm4.static.flickr.com/3065/2767999711_5eb4530dc7_b.jpg
To get this shot, I just sat him on my keyboard and 'tiddled' around with the cursor, this is what he's chasing.