Multisite Background Image

I have a wordpress multisite installation. Is it possible to share the same background image on all subdomains? I want to do this to speed up page load time. Currently, each site has the same image, but the image is uploaded to each individual media library; therefore, it reloads from a different file everytime I switch from site to site. Is there an easy way to write this into my main css file so that all sites pull from the same image file?