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To know whether thumbnails have been generated for a record, you can use the Search API and add has_thumbnail=true to your search criteria (preferrably in the qf parameter), or check if the edmPreview field in either Search or Record API responses have a value (which is the URL of the thumbnail).
Retrieving thumbnails from
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content resources
The Thumbnail API is able to retrieve a thumbnail for any media content resource that has been processed by Europeana.
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