Hmmm, I thought that you knew it, but the latest 1.0.3 version of the JA T3 Blank template made me sure you didn’t 🙂
In Joomla! 1.6, in addition to template_thumbnail.png you can include a larger screenshot of your template for preview. Name this file as template_preview.png and place it in the root directory of your template, where file template_thumbnail.png is located.
When you click a thumbnail in the ‘Template Manager: Templates’ screen, this larger preview image will open in a modal window. Its recommended size is 800×600 pixels. To be able to install it, please insert its row into the <files> section of the templateDetails.xml file of your template.