I did search the forum first before asking! :-) but didn't find answer i could do (there was one using the custom headers plugin, but i didn't understand it). I'm guessing it can be done with a CSS insert, but no sure what - thanks!

There is also the Dynamic Headers Plugin or you could add a new widget area in place of %image and then use a widget with conditional logic to only show on the front page. The conditional login could be by using the Display Widgets plugin or the Widget Logic Plugin.