I'm looking to create a little web app where one of the features is to
display a graphic--based on what a specific user has decided--that
looks as if it has been spray-painted over the page itself.

For example: AgentQ and MattMoney are using this; AgentQ wants to have
a penguin on the main page of Ubuntu. So he "boots up" the service,
heads on over to ubuntu.com, hits the "apply picture" button, picks
out the penguin pic, rotates it a little, and hits "Save." Matt boots
up the service and when he hits up ubuntu.com he sees Tux.

I was thinking it'd be possible with PHP+Javascript+mySQL or even a
Firefox add-on, but I will admit I'm WAY behind the times when it
comes to Web 2.0.

Let's take some examples.

There's a service that allows you to highlight websites and comment on
them and when others go to that site they can see the highlights, read
the comments, and leave their own comments.

There's also a passive MMO (PMOG.com) that uses a Firefox plugin to
create popups that have content created by its users.

Feel free to barrage me with questions as I'm sure I'm being vague
here.

BONUS COMIC!!
http://i74.photobucket.com/albums/i2...en/picpage.png