I'd like to ask this because I didn't find any information about it. As I saw the image service must be published to ArcGIS Server. I hope I'm wrong. Thank you for help!
You can only publish image services to ArcGIS Server (e.g., a GIS Server site). These services can then be registered as items in Portal for ArcGIS, and they can be used and viewed in Web AppBuilder for ArcGIS.
Hope this helps,
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