Hi Luca,
Thank you for your interest in Syncfusion Products.
Essential XlsIO do not have the direct support for auto fitting row to the image. We have created the sample with the work around for your requirement. Could you please find the sample from the below link. Try running the sample and let us know if this helps.
Sample link:
http://www.syncfusion.com/uploads/redirect.aspx?&team=support&file=InsertImage-700990036.zipplease let us know if you have any queries.
Regards,
poornima