Improve interaction with i18n_subsites plugin

[FriedrichFroebel]

When using the i18n_subsites plugin (https://github.com/getpelican/pelican-plugins/tree/master/i18n_subsites), the photos plugin will generate identical directories with the photos for each language. This does not make much sense if the image paths are absolute anyway (<a href="/photos/ ) and thus all languages will load the images from the primary language.