Luke as promised here is the issue I and at least one other user (from the XBMB3C thread) have been experiencing with regards to the Primary Image being scrapped as a Banner rather than a Poster. I did not initially report this due to running the server on OS X but now the other user has reported it too and they are using the server on Windows.
What I have found is that when TV shows are scrapped it creates a bunch of metadata and in particular a poster.jpg for the primary image. Initially this is fine, but through the night or whenever the daily schedule runs this primary image get's overwritten with a Banner resulting in the poster.jpg looking like it is suffering from low pixels. To circumvent this I have to go to each TV show and copy/paste the correct poster.jpg to another file called folder.jpg. With the folder.jpg file present the primary image no longer get's overwritten with the incorrect Banner image when the daily update occurs. The same principle applies to Movies too.
One other thing that happens with the OS X version (I have not tested to see if this occurs on Windows) is the CoverArt plugin takes a "slice" out of the artwork.