Not a major issue as the image does appear in the emails. However my hosting engineers have emailed me back after I requested a change to my php settings, and I quote:-
You may have a badly written plug in. It should be using relative notation to call not absolute reference. Always avoid using absolute as it can compromise your content if you allow outsiders directly URL access to folders containing content.
Security on the server is set to not allow direct/absolute calls which is why you are seeing the errors.