JohnMu spent a nice amount of time putting together a detailed FAQ on Instant Previews, which currently covers questions such as:
- What is Google Instant Previews?
- When are Instant Preview images generated?
- What’s the difference between on-the-fly and crawled previews?
- How can I influence the text highlighted in the preview image?
- Which browser can I test my pages with to check for compatibility?
- How often are previews updated?
- Can I show different content in the preview?
- How can I report broken or incorrectly rendered preview images?
- I use First Click Free for web search; do I have to change anything?
- How can I block previews from being shown?
- I want to block my images from being indexed, but I’m happy with them appearing on a preview image; how can I juggle the two?
- An Instant Preview snapshot is showing private content that I’ve removed from the webpage -- what can I do?
Now, if none of these answer your questions, then go to the Google Webmaster Help forum and post there.
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 (we're aware that Flash and other similar embedded technologies are not supported at the moment).
Forum discussion at Google Webmaster Help.