This feature is not supported in the RadSpreadsheet. We have a feature request for it on our feedback portal. You can track its progress, subscribe to status changes, and add your comment to it here: Spreadsheet: Add support for inserting images in header/footer.
I am afraid I cannot suggest a workaround or provide a timeframe for the feature implementation.
I want to apologize for the inconvenience this missing functionality is causing you.
Regards,
Dimitar
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