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?, Lion Mail’s Dock icon show unread only for one account., Having a icon problem, Can you change an Apple app icon?, iTunes Account Name Change.
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