The docbook.xsl file in epub directory contains multiple occurrences of "@role='front-large" to enable selection of an image type within mediaobject. The role value, rather than being hard coded to 'front-large', should be changed to a variable that is the value of preferred.mediaobject.role. This is because it lets the user choose the role value for selecting image types for the output format. For example, I have set the preferred.mediaobject.role in my customised stylesheets to 'epub' because it is a familiar term; I use this term for all imageobjects in several XML files. It would make sense and be more consistent to also use that value for selecting the cover image, rather than being constrained to using the value 'front-large', which would be inconsistent with all other imageobject role values.