change upload image url to images.domainname

I was using a separate server to handles all of my images with the url images.domainname.com, but since moving over to Nginx I am handles the www and images traffic on a single server. Google indexes my images at the images.domainname.com url, and it brings in a good portion of my traffic, so I’d like to keep it active.

I’ve been using a remote blog writing tool like Post2Blog, but would like to use the built-in post tool. The problem I have is that images posted only have the www prefixed to the domain name, but I would like the url to have images prefixed to it for images.domainname.com, which would preserve my Google indexing and allow me to migrate the images to another server if this server becomes overwhelmed.

Is there a block of code I could put into a plugin to accomplish the url change automatically?