Profile Pic/Siggy highlight text?

[Psychokitty]

I've noticed that in some people's siggys and Profile piccys that when you put the mouse over them, they show customised text instead of "*insert username here's* Profile Picture." And in the siggys, its the same thing, instead of showing "*insert username here's* signature," it has some words that the person put there. How do you do it?

e.g. Harle-Quin's siggy.