Would any of you be able to help with a query re this template.

The template is basically blue and down the left and right hand side of the Cart is a column in blue with a diagonal line across it giving a pattern.

We sell Tartan Items and would like to replace this "blue border" with a blue tartan border. I have a jpg picture of the Blue Tartan I want to use. What size of image should this be (my original is 1,000 pixels square) and exactly where in styslesheet.css do I need to change it.

THANK YOU.