Firefox not resizing images correctly

Hi,

This only happens in FF that I can see. If a large image (greater than 750 pixels wide) is used as the image for a single blog post with big preview image, then it does not resize it to fit across the post, but sprawls into the sidebar.

Similarly, if the image is 750 or less wide, then the resize is also not good, again going into the sidebar. You can see the issue at http://discoveringdowsing.com/polarity-reversal-how-to-fix-it/ where the image does not resize automatically after a certain dimension is reached.

Is this a deliberate action and is accounted for or can I alter that behaviour? Although I don’t intend to use very large images across the top of my blog posts, I’m concerned that there exist several such posts which do have them. I’d love to know the CSS I can use to make the image resize appropriately in all browsers.

Thanks!