update-vmss-deploy.json

{
  "$schema": "http://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "vmssName":{
      "type":"string",
      "metadata":{
        "description":"String used as a base for naming resources. Must be 3-61 characters in length and globally unique across Azure. A hash is prepended to this string for some resources, and resource-specific information is appended."
      },
      "maxLength": 61
    },
    "imageUri": {
      "type": "string",
      "metadata": {
        "description": "Uri of the custom VM Image"
      }
    }
  },
  "variables": {
    "namingInfix": "[toLower(substring(concat(parameters('vmssName'), uniqueString(resourceGroup().id)), 0, 9))]",
    "osDiskName": "[concat(variables('namingInfix'), 'osdisk')]",
    "computeApiVersion": "2016-03-30"
  },
  "resources": [
    {
      "type": "Microsoft.Compute/virtualMachineScaleSets",
      "name": "[variables('namingInfix')]",
      "location": "[resourceGroup().location]",
      "apiVersion": "[variables('computeApiVersion')]",
      "properties": {
        "virtualMachineProfile": {
          "storageProfile": {
            "osDisk": {
              "image": {
                "uri": "[parameters('imageUri')]"
              }
            }
          }
        }
      }
    }
  ]
}