RSS resources use relative URLs, they should not according to this:
http://validator.w3.org/feed/docs/warning/ContainsRelRef.html

An example image url from 4.5 master:
"/var/ezflow_site/storage/images/media/images/a1/515-1-eng-GB/a1_medium.jpg"

Setting "Site URL" when editing the RSS feed does not affect resource URLs, only link URLs. It should be used for both, I believe.

Steps to reproduce

• Publish an article with an image in the Summary field
• Create an RSS feed containing the article
• Note that the image URL is relative (BUG)