How do I edit the image shown in my iWeb blog summary?

The image in one of my blog posts is a large, portrait format photograph–and its fine. However, the cropped version of the image that shows up in my blog summary is not a good crop. It shows an uninteresting part of the photograph and I would like to be able to change that. Is there any way to move or edit the area of the photograph that shows up in the blog summary?