Provide separate image styles for Galleries thumbnail and Gallery thumbnail

Currently the media_gallery_thumbnail image style is used for both the Galleries and Gallery displays.

Media Gallery should provide separate image styles for each of these.

I have a use case where I want the Galleries display to use large images (say 200x200) but have the Gallery use a different thumbnail (e.g. 300x200, with a custom background (provided by the Imagecache actions module))