Check out how it works... hover over your name in the sidebar and your Google Profile pops up, along with your new picture:
Now that I've shown you how I pictures that don't belong to me, I'd like to point out Google's little note:
Remember, using others' images on the web without their permission may be bad manners or worse, copyright infringement.
I very much encourage you to find a picture which is truly YOUR picture! If anything, it's the best way to personalize YOUR Google Profile!