Handle images yourself (minimal implementation, add logic you might need here) — works on ASP.NET MVC as well as WebForms.

ImageHttpHandler.cs

using System;
using System.Linq;
using System.Web;

public class ImageHttpHandler : IHttpHandler
{
21public bool IsReusable
	{
		get
		{
			return true;
		}
	}

	public void ProcessRequest(HttpContext context)
	{
		var response = context.Response;
		var request = context.Request;

		var physicalFilePath = request.MapPath(request.Path);

		AddCachingHeadersToResponse(physicalFilePath, response);

		response.Clear();
		response.ContentType = GetContentType(physicalFilePath);
		response.TransmitFile(physicalFilePath);
	}

	private static void AddCachingHeadersToResponse(string physicalFilePath, HttpResponse response)
	{
		response.AddFileDependency(physicalFilePath);
		response.Cache.SetETagFromFileDependencies();
		response.Cache.SetLastModifiedFromFileDependencies();
		response.Cache.SetCacheability(HttpCacheability.Public);

		response.Cache.SetExpires(DateTime.Now.AddMonths(1));
		response.Cache.SetMaxAge(new TimeSpan(30));

		response.Cache.SetSlidingExpiration(true);
		response.Cache.SetValidUntilExpires(true);
		response.Cache.VaryByParams["*"] = true;
	}

	private static string GetContentType(string physicalFilePath)
	{
		var extension = physicalFilePath.Split('.').DefaultIfEmpty(string.Empty).Last().ToLowerInvariant();

		switch (extension)
		{
			case "jpg":
			case "jpeg":
				return "image/jpeg";
			case "png":
				return "image/png";
			case "gif":
				return "image/gif";
			default:
				return "application/octet-stream";
		}
	}
}

Web.config

<configuration>
  <system.web>
    <httpHandlers>
      <add verb="GET,HEAD" path="*.jpg" type="Website.Handlers.ImageHttpHandler,Website" />
      <add verb="GET,HEAD" path="*.gif" type="Website.Handlers.ImageHttpHandler,Website" />
      <add verb="GET,HEAD" path="*.png" type="Website.Handlers.ImageHttpHandler,Website" />
    </httpHandlers>
  </system.web>
</configuration>