Unfortunately, you cannot publish an image service to ArcGIS Online. To publish an image service, your ArcGIS Pro project must contain a publisher or administrator connection to a stand-alone ArcGIS Server 10.6 or later site. See Publish an image service for more information.
In the past we have created a map tile package to publish aerial imagery as a hosted tile layer (without Enterprise). Would this work for your needs, or do you specifically need to publish an image service?