+ 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)