Header Highlight Content Image Size.

Hi, I have looked everywhere and I cant find how to set the default image size in header highlight to not exceed the size of the group song demo pic. An example here cinemaassassin.com The main header image is a poster for NACIYE. This is about 3-4 times larger than I wanted taking up way too much real estate while at the same time being extremely blurry due to the image being oversized. Please help.