To help fine-tune the preview generation, it would be useful to have URLs of pages where the images are either not appearing, or incorrect in some form. If you see suboptimal, missing or broken preview images for your websites, we would appreciate your posting the URLs here so that we can take a look. The most common issue that we've seen so far comes from blocking parts of your website from being crawled.
With Webmaster Tools, it's possible to check a few things in that regard:
You can check for crawl errors that may be affecting content that's used to generate the preview
You can double-check that the appropriate URLs can be crawled with the robots.txt tester