Thanks very much for the reply; I'll give Xvfb a shot.
Should this situation--running ebook-convert at the console without any X environment be warned against, or should any error flags be thrown? I'd be happy to put together a patch if you think it would be a good idea.
Alternatively, would it be a good thing to add a "don't modify cover" flag to be used optionally, or in a fallback situation if there's no X? Could be a totally stupid question, so apologies if so!
Thanks again--calibre is a very impressive software package.