Post background won't fill behind image

When I write a post with just an image in it, the post’s background color doesn’t fill behind the image. It will only fill behind text. On posts where I’ve put large images that fill the whole width, if I put text after the image, the background stretches. While I’ve figured out that workaround, I would like to solve the real problem.

Here is the page where this problem is visible:
https://www.marconiband.com/news/

It’s not too noticeable right now, but there should be 20px padding on the bottom of the post below the alignleft image. But the background isn’t stretching the full 20px because it won’t recognize the image. If the text on the right didn’t fill the whole height of the image, the background would end wherever the text ended, leaving the image dangling.

I designed this as a child theme of Thematic. Thanks for any input!