Help with Valentine's day special worm

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?

This is not an appropriate forum for you. All your posts are about writing malicious code, something we all seriously frown upon. Just go away.

This forum is for computer programming professionals, not for malware writers. This type of questions is not condoned.