We just fixed a long-standing bug on our WordPress network. Until now, individual page header images were sized much smaller than they should have been (i.e. – 880px wide). Evidently a remark was present in functions.php that based image header thumbnail creation on height rather than width. Why it was written like that remains a mystery, but rest assured the problem has been permanently fixed.
Site Admins: if you use special header images on any of your pages, you’ll have to rebuild your site’s thumbnails or the fix won’t be reflected on your end. To do this, activate the “Regenerate Thumbnails” plugin (if it isn’t already active), then click Tools > Regenerate Thumbnails. All thumbnails on your site will be created accodring to current specs (which include the fix). Now all your headers should display at the appropriate size.