Use correct variable. fixes #24014. see #23119.

git-svn-id: http://core.svn.wordpress.org/trunk@23941 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: better representative theme screenshot, props Joen. (Also shaves a few KB off previous file.)

git-svn-id: http://core.svn.wordpress.org/trunk@23942 1a063a9b-81f0-0310-95a4-ce76da25c4cd