On Lubuntu login screen, one might see a picture representing each login, here is the way to change this picture.
On Ubuntu, the user management tool provides a way to modify the avatar picture for each login. The user management tool from Lubuntu does not provide this function.
To change it, you only have to change the file ~/.face (a square PNG file - size 96x96 pixels) to whatever picture you want ... and do not forget to give read rights to every one.
Et voilà, you new avatar is here ! !