How can I change the Facebook preview image for my homepage?

Your logo is set as the default image used for the preview image in Facebook. However Facebook requires the preview images be at least 200px by 200px. If your logo is smaller than that, then Facebook will use another image on the page that fits its size requirements, or simply not show a preview image at all.

You can always explicitly specify what image is used as the preview image in Facebook and other social media by adding it as the "Featured Image" the page editor in the dashboard.