Shopify background fixed?

Hi there,
I have uploaded a picture as the background for my shopify theme, however if i make my browser window smaller the background stays the same size and doesnt resize with the window. Any help would be appreciated.
Hi Chris, you wil need to use JQuery for this, try google searching - jquery.dimensions.js - i've used this one previously and it worked for me on a normal site to shopify should acept the code