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You can add to this information by declaring a class which inherits from Summary and associating that with your content type. Lynicon supplies a class called ListingSummary which includes a thumbnail image. It is recommended that any information about a content item that might be needed when viewing a page not associated with that item is put into a subtype of Summary.
The reason for this is that the summary system is used to enhance performance in the Data API System by only fetching the necessary fields to build a the summary object when that is all the client is requesting.
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Finally, a common caching strategy is to use the SummaryCache Summary Cache module to store the summaries of all content items in the system. As summaries contain relatively little data, this is an efficient trade-off which allows any data access over and above the content record for the current page to be done in memory hugely increasing site speed. Even for a site with thousands of pages, the summary data can be held in a few megabytes of cache memory. It also allows for efficient querying to be done with the Content persistence model despite the database records storing serialized data.