I'm doing some Responsive HTML5 output, and testing the results for Desktop/Tablet/Phone by resizing the browser window, and I've noticed that;
I've actually just resolved this, so for the sake of posting the solution...
The culprit was the underlying html associated with some italicized text in the table. On the surface, the text looked fine. But in the HTML it looked like this:
I have no idea how that's happened, or why that alone prevented the image from resizing, but cleaning that up has fixed the issue.