[HM666]

That can be accomplished with CSS although I think you might be able to set that within the Stylevars as well.

If you want to do this with the main background of your web site which is what I'm assuming since you did not specify you can add this to your additional.css template.

html {
background-image:url(path/to/your/image);
background-position: bottom left;
}

Change the path/to/your/image to the actual path to your image and then save the template.