Can't set login image

I have used ubuntu tweak before on older versions of Ubuntu and follow the same basic steps.

What I did:

Go to applications, select system tools, open system settings and click on the tweak icon.

In the main window I clicked on the login settings link on the upper right hand corner of the window, which launched a four tab window on the tweaks tab.

Then I clicked on the unlock tab, entered the admin password to unlock the window.

I then clicked on upper login screen button which brought up a window to select an image from my home folder.

I can move between folders i.e. home folder and my pictures folder, but I noticed that the application only "sees" PNG files.

Previously I had used the right click menu on the desktop to set my desktop Image. This will become important later.

What happened:

I selected an image for my login screen rebooted and discovered this:

The login image was of my desktop with a band that I later found was a snippet of the login image I had selected.

Unsatisfactory work around:

The only way I can get a login image is to set my desktop through Ubuntu to an image and then set both of the settings on the tweak login images to the same image I am using for the desktop.

What I expect to happen:

What I expect is that I can set the desktop to one image and set the tweak login screen settings to another image and have the respective images to appear at the right time in the right place and appear correctly as well as being able to select whatever image I want i.e. SVG, PNG, JPEG etc.

The system I am using:

I am using Ubuntu 12.04 LTS 64 bit with a classic Gnome desktop setup on a triple boot system (Windows Proffesional and Windows Premium) on a Dell G62-227 CL notebook. Nothing else seems to be "wrong" at this point.

I don't think I am doing anything wrong, but if I am tell me what I need to do to fix this issue.

Thanks,

Mike