MediaHelper.cs

using Sitecore.StringExtensions;
namespace SOPA.Framework.Helpers
{
    using Sitecore.Data;
    using Sitecore.Resources.Media;
    public static class MediaHelper
    {
        public static string GetImageUrl(string imageSrc, int width, int height, bool allowStretch, bool protectAssetUrl)
        {
            if (imageSrc.IsNullOrEmpty())
            {
                return $"http://placehold.it/{width}x{height}";
            }
            var imageUrl = $"{imageSrc}?w={width}&h={height}&as={(allowStretch ? "1" : "0")}";

            if (protectAssetUrl)
            {
                return HashingUtils.ProtectAssetUrl(imageUrl);
            }

            return imageUrl;
        }
    }
}

Get sitecore media url