Images in subdomain?


I'm using Shinkansen for a little while and it's awesome. However, some new requirement came up and I was wondering if there is a way to accomplish it with Shinkansen.
As of today I use images hosted in my full domain (www.domain.com), but now I need to serve them from a subdomain (img.domain.com). Since my images are all using relative path (../img/sprite.png) and the Shinkansen handler is served from the full domain I'm not sure this is possible. I thought about using the full subdomain url for the images inside css (http://img.domain.com/sprite.png) but if I hardcode the HTTP protocol I will have problems in my HTTPS pages.
Another option would be to remove Shinkansen and use a post-build process to minify and unify css/js but I don't really want to do that.
Any suggestions on how to do that?
Thanks a lot!