Blogs MU theme not formatting properly on tablets?

The header image isn't responsive -- doesn't scale properly when viewed on a tablet (e.g. ipad) or cellphone. On cellphones the image is too small, on tablets the header image is displayed too large (e.g. the length hangs far to the right relative to the alignment of the rest of the page). I'm most concerned about the tablet issue, but would appreciate a fix for both. Does anyone know css well enough to help me with this? Thanks!