You can find the default pictures of Windows 7 user Account in a hidden folder, path given below. Also you can find Admin & guest account picture here.
? The default Pictures are located in the hidden C:\ProgramData\Microsoft\User Account Pictures\Default Pictures folder.
? The default Standard and administrator account picture is the hidden C:\ProgramData\Microsoft\User Account Pictures\user.bmp file.
? The default Guest account picture is the hidden C:\ProgramData\Microsoft\User Account Pictures\guest.bmp file.

If you wish to change these simply replace them with another file up to 128 x 128 pixels in size to prevent it from being automatically cropped to fit the user picture display box.