defensive check for relative path of image destination

In my EPUB workflow, I am setting the resource_directory to an absolute path, which propagates to absolute paths in the image generation. That breaks the moment the site directory is turned into an archive, as the paths change. So there is a need to keep them relative.

I am also considering changing the resource_directory to a relative path, I used an absolute one just to be sure it was pointing at the right location initially. Feel free to refuse the request if you think that would be the right patch. Though defensive programming almost never hurts.