I am struggling with the same issue, The GPTask keeps returning the error: esriJobMessageTypeError: Invalid value for parameter Input Multidimensional Raster (Image Service url) - Details : Unable to parse JSON value of the parameter.
Maybe it needs to be in JSON format
{ "url": "https://myserver/arcgis/rest/services/crf_rcp85_all_data/ImageServer" }
https://developers.arcgis.com/rest/services-reference/enterprise/gp-data-types.htm