lock screen ubuntu 11.10

Under the menu in the top right (I assume you use Unity shell) you can lock it there..

i have tried but it wont do anything ?? is there a way i can reset using terminal ??

Maybe open system-settings from panel and go to keyboard -> tab shorcuts -> system, lock screen should be ctrl+alt+L and can be adjusted to preferred shortcut.

> change my background on my login screen i downloaded a tweak but now it it just dots when i set the picture cant even put it back to default please help

Please provide steps how you did that in order to reset.

Oneiric is using unity-greeter: /etc/lightdm/unity-greeter.conf
Which incl. the default background, the relevant line is:
[greeter]
background=/usr/share/backgrounds/warty-final-ubuntu.png

Replace it to preferred path.
background=<path_to_my_custom.png>

Logout-in.

## Note, on comming Precise 12.04 this isn't necessary, unity-greeter is able to use the desktop-background which is selected by the user from user-interface in system-settings.

i was using lightdm and changed the photo and does not show

It works here, maybe you could describe which steps were taken and post content of .conf files.
Another description there.
http://www.webupd8.org/2011/09/how-to-change-lightdm-login-screen.html