As of version 3.1.6+ of all themes, the header displays at whatever image it is uploaded at. Editing your theme code is no longer required.
For all update notes, see the changelog at the bottom of the theme page at:
Header images are set in the Admin > Appearance > Customize > Header Image screen.
In all of our themes version 3.1.6 and up, header sizes are displayed at whatever resolution they’re uploaded at. This tutorial is outdated for versions newer than 3.1.6. See this post: rethinking the header image.
If you’re running a version before 3.1.6, please update to the latest version of the theme, as we no longer support those older versions.