I have saved .rft.xml files from ArcGIS Desktop Symbology workflows and imported them into My Content on ArcGIS Online. However, when working with my maps in ArcGIS online and then going to the Image Display properties for the Image Service I want to render with the template, I do not find a way to add or access the templates I have uploaded. What is really puzzling is that I see a template that I must have made a while back (the 2018HydroImage template seen in the screen grab below), but I have no notion of how it got there. I don't see it in My Contents. Any advice appreciated.
You may have solved this but I'm posting anyway. Raster functions are exposed in ArcGIS Online by including them using the function tab while publishing the image service. The function tab will let you navigate to your saved rtf.xml files and set a default for the service if you want to. The attached image shows their availability in ArcGIS Online if you push up the raster functions with your service. Doing this also makes the raster functions available for popup configurations. The popup in this app shows the elevation in feet that comes from a raster function. I hope this helps (if you still need it).