It actually probably isn't possible without hand coding. The problem is that Image Resizer will copy and cache the image referenced and update it to a relative url. In the case of sending an email you need to use a full http:// address to the image in the src.
In my opinion, the best option would be to create a copy of the image resizer include file and then manually add the http:// address for that site to the php code where it updates the source.