UPDATE: Looking at the page source, I see that after the closing of the header tag, there are 3 divs:
<div style=”margin: 0 auto;”>
<div style=”position: relative;”>
<div style=”width: 600px; margin: 0 auto;”>
If I remove the style=”width: 600px” code, the problem is fixed, and the image becomes responsive.
How can I modify this theme to not have it generate this code? There is no hard-coded HTML I can find to edit.