Remove old homepage images, optimise current ones

The old images don't seem to be being used by anything and just add to
confusion when looking at the folder.

Shrink the vehicle-tax image down to the largest size it will be needed
at so the homepage loads faster.
Update homepage promo to warm this winter

Update the world slot on the homepage to the warm this winter campaign.
Merge pull request #685 from alphagov/update-homepage-image

Update homepage promo. For deploy 20th October.