You can change your account icon by going to System Preferences, Accounts. Then click on your icon to select a new one, or choose Edit Picture to select a file or use your iSight camera to take a new picture. You can also drag and drop from the finder onto the icon. Or, you can select the icon and paste into it. You will then have a chance to zoom in and recenter.
Related: How do I change icon for folder holding all my camera apps?, Copy and Paste Files, Safari Upload Browse Button, Lion Mail’s Dock icon show unread only for one account., Get Email Account Info.
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