Pictures included in spreadsheet increase the size of the stored file on an inacceptable way. Example: 8 pictures (JPG) have together 5,6 MB size. Adding them to a spreadsheet and storing produces a xlsx file with 9 MB or PDF file with 13.3 MB. Missing image optimisation limits essentially the use of gembox program.
This feature request has been implemented and is available in the latest versions of GemBox.Spreadsheet.