How do I change the small photo that appears in the password prompt box when logging in to my iMac?

Hi,

I hope someone can help me over a triffling but irritating matter. My son has somehow put a pic of himself - I think taken from the photobooth - in the password login box and I want to remove it but I can't find how to do it in system preferences. This happened to me when I first got my mac 2 yers ago, but I can't for the life of me remember how I removed/changed the samll personal pic. Wouldbe most grateful if anyone could advise me! Many thanks.