I am not sure if I am just missing a step or option somewhere, but I have noticed that if I have a particular show that I have run "fix incorrect match" and changed it the use TMDB to pull in the metadata, afterwards, if I then try and update the image it will only pull options for the images from TMDB, therefore leaving only the option to choose a poster.
The only way to work around it is to change the "fix incorrect match" back to Gracenote, then the update image option pulls from Gracenote and you can then select a 4:3 image and then go back again and run "fix incorrect match" and change it back to TMDB to get the correct metadata again for the episodes.
It will at least retain the last image that was selected, so the 4:3 image stays after you do all these steps.
Am I missing some other way of doing this or is it a bug?