Just click on the image currently shown and in the window that appears, choose "Change your picture"

You can select from the images shown or you can click "Browse for more pictures…" and choose any image you like. Don't worry about the image size, Vista will size it as needed (no, it will not modify the original). Pick a picture of yourself or something you like more than an orange flower (unless that's what you are into).

Note, this same image also appears at the logon screen when you start your system.