make a picture with the same color as the logon/theme screen you have and then select that as your account picture that way it will look as if there is no picture, as far as i know there is not a way to disable it entirely.
There is no user picture in start menu if you use Windows classic look. Probably not what you want.
There is also no user picture if you uncheck "Use Welcome Screen" in control panel - user accounts - change the way users log on.
Choosing a transparent user pic or one matching the background of the title bar will still leave the frame around the picture. Only thing to avoid this I can think of would be to edit the msstyles file(s) you are using.