Revert "Merge pull request #1537 from alphagov/relax-inline-image-size-r...

...estriction"

It was decided that images should only be 960*640px on GOV.UK.

This reverts commit 73b76fd98aa9e2bbf2bd9a6ccd7bfef4296dbbec, reversing
changes made to f392ad4f58ea25b8c6ff62337dec25c5035904d3.