Can I align/lock the login background to a screen edge?

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http://askubuntu.com – I have used Ubuntu Tweak to set a custom login screen background, which works well. I would now like to instead set a very wide image as the desktop background, with the following layout properties: adjust the size of the image to fill the height of the screen adjust the width of the image proportionally fix the right edge of the image to the right edge of the screen Thus, the image would remain proportional, but would overflow the left side of the screen. On wider screens you would see more of the image. Can this be done? (HowTos)