<img src="/barchart.aspx?width=1024&height=768&chartId=50" >

public class ImageGeneratorController : Controller {
    public ActionResult BarChart(int width, int height, int chartId) {
        // ASP.NET MVC will map the request parameters to method arguments
    }
}

Url.Action("BarChart", "ImageGenerator", new {
    width = 1024,
    height = 768,
    chartId = 50
});

/ImageGenerator/BarChart?width=1024&height=768&chartId=50

public FileResult BarChart(int width, int height, int chartID) {
    //create the chart
    return new FileContentResult(byte[] fileContents, string contentType);
}

<img src="/yourController/BarChart/1024/768/50">