Azure Functions is the most appropriate service for this scenario as it allows developers to execute code in response to various triggers, such as the uploading of images to a storage container, without the hassle of managing infrastructure. It automatically scales the execution to match the demand and you incur costs only when your functions are running. Azure Event Grid is an event-routing service that could help trigger the function execution but, by itself, does not handle code execution. Azure Container Instances (ACI) can quickly run containers without requiring server management but it's not inherently serverless and lacks the automatic scaling based on events. Azure Automation primarily targets automation of cloud management tasks rather than being a serverless platform for running custom code in response to events.