When uploading new (or existing) images to the media library in WordPress 3.6.1 the preview generated on the admin page/edit media page (eg. wp-admin/post.php?post=8736&action=edit )
is showing one of my custom image sizes defined from my functions file, rather than the default size.

I am using:

add_image_size( 'narrow', 205, 450, true );

In functions.php with after_setup_theme

This is an issue because this size is for part of the theme and is cropped so does not show the image as intended.

Question: Why is WordPress choosing a custom image size on the editor and how can I force it to use a standard image size such as ‘medium’.

Thanks