在ASP.NET Core中，可以通过自定义中间件来返回图像。以下是一个示例代码，展示如何使用中间件返回图像：

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using System.Drawing;

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        // 添加所需的服务
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        // 配置中间件
        app.UseRouting();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapGet("/image", async context =>
            {
                // 创建图像
                Bitmap bitmap = new Bitmap(200, 200);
                Graphics graphics = Graphics.FromImage(bitmap);
                graphics.Clear(Color.Blue);
                graphics.DrawString("Hello, Image!", new Font("Arial", 12), Brushes.White, new PointF(0, 0));

                // 将图像以流的形式返回给客户端
                context.Response.ContentType = "image/png";
                await bitmap.SaveAsync(context.Response.Body, ImageFormat.Png);

                // 释放资源
                bitmap.Dispose();
                graphics.Dispose();