Gist's for the Marcel Digital Blog post about creating a ImageGen HTML Helper.

HelperExampleCrop.cshtml

<img src='@Umbraco.ImageGenUrl(Model.Content.GetPropertyValue<int>("image"), new { width = 200, height = 270, crop = "resize" })'
     alt="Image" />

HelperExampleOverlay.cshtml

<img src='@Umbraco.ImageGenUrl(Model.Content.GetPropertyValue<int>("image"), new { text = "Marcel%20Digital", fontcolor = "aqua", align = "center" })'
     alt="Image" />

HelperExampleRotate.cshtml

<img src='@Umbraco.ImageGenUrl(Model.Content.GetPropertyValue<int>("image"), new { rotate = 30 })'
     alt="Image" />

ImageGenHtmlExample.html

<img src="/imagegen.ashx?image=/img/image.png&width=580&height=270&crop=resize" 
     alt="Image"/>

ImageGenRazorExample.cshtml

<img src='@Umbraco.ImageGenUrl(Model.Content.GetPropertyValue<int>("image"), new { width = 580, height = 270, crop = "resize" })'
     alt="Image"/>

UmbracoHelpers.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Routing;
using umbraco.NodeFactory;
using Umbraco.Core.Models;
using Umbraco.Web;

namespace MarcelDigital.Umbraco.Helpers
{
    public static class UmbracoHelpers
    {
        /// <summary>
        /// Creates the url from the media id to run an image through the image gen plugin
        /// </summary>
        /// <param name="helper"></param>
        /// <param name="imageId">Umbraco media id of the image</param>
        /// <param name="options">Options to pass to the image gen plugin</param>
        /// <returns></returns>
        public static String ImageGenUrl(this UmbracoHelper helper, int imageId,
                                                                    object options) {
            string imageUrl = helper.Media(imageId).Url;

            return helper.ImageGenUrl(imageUrl, options);
        }

        /// <summary>
        /// Creates the url to run an image through the image gen plugin
        /// </summary>
        /// <param name="helper"></param>
        /// <param name="imageUrl">Url to the image that needs to be proccessed</param>
        /// <param name="options">Options to pass to the image gen plugin</param>
        /// <returns></returns>
        public static String ImageGenUrl(this UmbracoHelper helper, string imageUrl,
                                                                    object options) {
            var baseUrl = "/imagegen.ashx?";
            var optionsDictionary = new RouteValueDictionary(options);
            optionsDictionary.Add("image", HttpUtility.UrlEncode(imageUrl));

            return baseUrl + String.Join("&", optionsDictionary.Select(o => String.Concat(o.Key, "=", o.Value)).ToList());
        }
    }
}

Web.config

<system.web.webPages.razor>
  <host factoryType="System.Web.Mvc.MvcWebRazorHostFactory, System.Web.Mvc, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
  <pages pageBaseType="System.Web.Mvc.WebViewPage">
    <namespaces>
      ...
      <add namespace="MarcelDigital.Umbraco.Helpers" />
    </namespaces>
  </pages>
</system.web.webPages.razor>