I have Kubuntu 13.04 (last version) and lightdm

I want to change my login screen background (not my wallpaper after I type my password. I'm talking for the image you see WHEN you type your password).
It shows a greyish "image" right now.
When I tried to use a picture from home and restarted the screen became white and it didn't show my image. I searched and found out that it because of some setings.

The problem is I'm very new to kubuntu and I don't want to mess things up with complicated setting from the terminal etc or to mess aroud with the root files because I won't be able to fix things.

Is there a simple solution as to how can I change this image? There is no option for me to download something there but only to load a picture.

Thank you very much in advance