This might be dumb but...me and my friend here are trying to make our girlfriends a surprise on Valentine's day.

We tought at this: we need to change their wallpaper on their pc to a wallpaper chosen by us(telling go at spot x at hour x)

And the point is,I can't get acces to any of the computers,the girls are actually both using only 1 router.

So I tought at making some kind of virus/worm,that will be sent to both like this:

I go in their house with my laptop,I use wifi to connect and launch that virus/worm and that's it.

The problem is that I don't have any ideea on how to make a worm or virus,especially one that only changes the wallpaper...

BTW FORGOT: They're not using any antivirus.

Sorry for bad english...

I know this sounds childish but...If I'm learning programming,at least I can make a better surprise don't i?