1、完美解决方案，支持PNG透明格式

using (FileStream fs = new FileStream(path, FileMode.Open))
{
byte[] byData = new byte[fs.Length];
fs.Read(byData, 0, byData.Length);
fs.Close();
context.Response.Write("data:image/jpeg;base64," Convert.ToBase64String(byData));
}

案例：cloud->ashx->base64

2、只能生成JPG格式，PNG背景为白色，不推荐使用，仅供学习System.Drawing.Bitmap用法。

System.Drawing.Bitmap bmp = new System.Drawing.Bitmap(path);

MemoryStream ms = new MemoryStream();
bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
byte[] arr = new byte[ms.Length];
ms.Position = 0;
ms.Read(arr, 0, (int)ms.Length);
ms.Close();
context.Response.Write("data:image/jpeg;base64," Convert.ToBase64String(arr));