Image thumbnail generation

Design a service where users upload a profile photo and the system produces a few resized thumbnails (e.g. 50px, 150px, 300px) for use across the app. Uploads are modest, a few thousand a day, but resizing shouldn't make the upload request slow, and the thumbnails must be available shortly after upload. Cover the components, how resizing happens, and how the app knows when thumbnails are ready.