AWS Lambda is a serverless compute service that automatically scales to match the demand of triggering events and charges on a per-request basis. This makes it ideal for handling unpredictable workloads such as the image processing required by the mobile application during spikes in activity, without incurring costs during idle times. Amazon EC2 would require manual scaling and constant running costs regardless of demand. AWS Fargate, while it provides container management, still requires capacity management and potentially suffers from cold starts. AWS Batch is meant for batch processing workloads and lacks the real-time event-driven model needed here.