Urgent- mobile-responsive header help, please!

Hello,

Please help- I’ve just launched a site and the theme designer is unresponsive right now; but I need to quickly fix some responsive design issues for mobile devices quickly, so I’m making this post general. (You can see how the design looks on various devices here.)

The most important issue is that I need to change how the banner displays on different devices. Right now, the majority of the banner just gets lobbed off on most phones, and even on an iPad.

Ideally, I would like to have a different banner image load depending on the size of the device, however I’m open to better ideas. Also I’d like to make this edit in the theme’s skeleton.css, as this seems the neatest place to do it, but I don’t know exactly how! Help?!

Thanks,
Nicole