increase screenshot size to 880x660 to 3.8 theme standards
Merge pull request #382 from mrwweb/fix_screenshot

_s: Increase screenshot size to 880x660px.
Merge remote-tracking branch 'upstream/master' into ja

* upstream/master:
  increase screenshot size to 880x660 to 3.8 theme standards
Merge remote-tracking branch 'upstream/ja'

* upstream/ja:
  increase screenshot size to 880x660 to 3.8 theme standards