[andrewilley]

Use the Choose Art option in the Information Page of the TV Show. The show needs to be scraped/scanned into the library to do this. 

  
That was my point, the lack of that option seems to make it impossible to add or change cover artwork for a group/folder that you've manually created in TV Shows (for example to contain assorted .nfo based items which cannot be scraped, which is the case here). Choose Art simply returns an error. The main time users would need to browse for local artwork images would be where nothing can be scraped online, so to require the content to have been scraped before a manual file Browse option becomes available seems illogical.

Anyway, I resolved it by reverting to the regular default Estuary skin, browsed there for my new image file (I created a new 2:3 ratio portrait version so the the MOD skin would not later squash it) and then switched back to Estuary MOD. I do still think that the default skin's display behaviour of cropping images rather than distorting them is the better fall-back for wrongly shaped artwork, but it's an easy enough fix in a image editor. 

Andre