Add compat code for 'image' post format.

see #19570. props wonderboymusic.

git-svn-id: https://core.svn.wordpress.org/trunk@23847 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: simpler name for custom image size for image post formats, see #23620.

git-svn-id: https://core.svn.wordpress.org/trunk@23848 1a063a9b-81f0-0310-95a4-ce76da25c4cd