In my other text Dynamic list of Jasper reports based on jasperserver internal database you can see how I get list of reports with their thumbnails from JS internal database.

To use these thumbnails in report you must select the column in report’s Dataset into field of Class Type “java.io.InputStream”. This field you use in “Image” element as “Expression” with evaluation time “auto”.

Link: Step by step guide for adding dynamic images from database to jasper report