Here’s one tweak tip to customize your Windows 7 experience. We’ll add a customized picture or an image you really like for your Windows 7 user account. First, using a photo editing software, edit your image to make it have a 128 x 128 pixels dimension and save it as a bitmap (BMP). Move your edited image to:
C:\ProgramData\Microsoft\User Account Pictures\Default Pictures
Using the User Accounts applet on your Control Panel, you may now browse for your customized image to use it as your account photo.
Below is a screenshot of my User Account applet as I change my account image.
You may save your edited photo as JPEG but Windows will not display a thumbnail image of it. I think this also works for Windows Vista.