Make sure that you're logged in to the user account whose picture you want to change. From the Apple menu, select System Preferences...
Move your mouse cursor over the account picture and click. From the popup menu that appears select Edit Picture.
Drag any image file from your Desktop or a Finder window onto the resulting Images window.
Use the slider at the bottom of the window to zoom in to your picture. You can also drag the image around in the window to adjust the framing.
When satisfied, click Set to make the change.