Login to ArcGIS Online > Navigate to Content tab > Click on 'New Item' button > Select Imagery Layer option > Upload GeoTIFF. This workflow is outlined in this documentation: https://doc.arcgis.com/en/arcgis-online/manage-data/publish-imagery-layers.htm.

Please see https://gis.harvard.edu/arcgis-online on how to login to Harvard's ArcGIS Online organization using your HaervardKey. In case your ArcGIS Online account does not have the ArcGIS Image extension, please request it by filling out this form: https://gis.harvard.edu/contactus.