ConvertUploadPipline.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace v3 {

	class Program {
		static async Task Main(string[] args) {
			var w = new Worker();
			var links = await w.Start();
			Console.WriteLine("Uploaded to cloud files URLs:");
			foreach(var linkItem in links) {
				Console.WriteLine(linkItem);
			}
		}

	}
	class Worker {
		public async Task<IEnumerable<Uploaded>> Start() {
			await Task.Delay(0);

			var proc = new ImageProcessor();
			var resizedImages = proc.GetResizedImages("sourceFile.jpg");

			var tasks = new List<Task<Uploaded>>();
			foreach(var image in resizedImages) {
				var uploadTask = Task.Run(async () => await proc.UploadImage(image));
				tasks.Add(uploadTask);
			}
			await Task.WhenAll(tasks);

			return tasks.Select(x => x.Result);
		}
	}

	class ImageProcessor {

		public IEnumerable<Resized> GetResizedImages(string fileName) {

			for (var i = 0; i < 10; i++) {
				Console.WriteLine($"Start resize image {i}");
				/// Имитируем работу конвертора изображений
				var rand = new Random(DateTime.UtcNow.Millisecond);
				Task.Delay(rand.Next(100, 400)).Wait();


				yield return new Resized(i);
			}
			yield break;
		}




		public async Task<Uploaded> UploadImage(Resized image) {
			Console.WriteLine($"Start upload image {image.Index}");
			/// Имитируем работу загрузчика на облако
			var rand = new Random(DateTime.UtcNow.Millisecond);
			await Task.Delay(rand.Next(400, 700));

			return new Uploaded(image.Index);
		}


	}

	class Resized {
		readonly int stream;

		public Resized(int index) {
			this.stream = index;
		}

		public int Index { get => stream; }

		public override string ToString() {
			return $"{nameof(Resized)} file - STREAM: {stream}";
		}
	}

	class Uploaded {
		readonly int url;

		public Uploaded(int index) {
			this.url = index;
		}

		public int Index { get => url; }

		public override string ToString() {
			return $"{nameof(Uploaded)} file - URL: {url}";
		}
	}
}