Try System Settings - User Accounts. Click on the grey head (or the thumbnail ) that's shown above 'Account Type'. There you can change it.
But...a default Ubuntu 13.04 installation will not show a picture of you in the lightdm login screen.
There's information on Help Ubuntu and on this ArchWiki page on how to customize lightdm.
You need to edit the
/etc/lightdm/lightdm-gtk-greeter.conf
This is how a fresh installed lightdm-gtk-greeter.conf file looks like:
#
# background = Background file to use, either an image path or a color (e.g. #772953)
# theme-name = GTK+ theme to use
# font-name = Font to use
# xft-antialias = Whether to antialias Xft fonts (true or false)
# xft-dpi = Resolution for Xft in dots per inch (e.g. 96)
# xft-hintstyle = What degree of hinting to use (hintnone, hintslight, hintmedium, or hintfull)
# xft-rgba = Type of subpixel antialiasing (none, rgb, bgr, vrgb or vbgr)
# show-language-selector (true or false)
#
[greeter]
background=/usr/share/backgrounds/warty-final-ubuntu.png
theme-name=Ambiance
font-name=Ubuntu 11
xft-antialias=true
xft-dpi=96
xft-hintstyle=slight
xft-rgba=rgb
show-language-selector=true
file.