* Add in a middle image size, so thumbs -> medium -> full size (and option for
  this)
* Allow to recreate galleries and use new captions (only resizing changed
  images) - mostly done, needs to be able to check and change based on
  the width specified then it'll be finished
* Allow a fixed height to be specified instead or as well as a fixed width, in
  the latter case, resize to which ever fits the bounding box, but keep the
  aspect ratio.
* Make sure that the tools we require are available (i.e. check that we have
  xargs, convert, identify etc)