Twenty Thirteen: fix translation string in image template, fixes #23799.

git-svn-id: https://core.svn.wordpress.org/trunk@23758 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: provide max-width override for gallery thumbnails to collapse to two columns in small screens. Props kwight, fixes #23762.

git-svn-id: https://core.svn.wordpress.org/trunk@23759 1a063a9b-81f0-0310-95a4-ce76da25c4cd