Is there a way to set a common image path for LESS files?

Try using string interpolation for things like this. Look for “variable interpolation” in docs.

@base-url: "http://assets.fnord.com";
background-image: url("@{base-url}/images/bg.png");

The solution:

.side-bg
{
    background : ~"url( '@{image-path}/layout/side-bg.jpg' )" top no-repeat;
}