private void Start() {
    string path = "./aaaa.png";


    // 1　var i = Image.FromStream(f)でクラッシュ

    using(var f = new FileStream(path,FileMode.Open,FileAccess.Read))
    using(var i = Image.FromStream(f))
    using(var b = new Bitmap(i))
    using(var m = new MemoryStream()) {
        b.Save(m,ImageFormat.Png);
    }


    // 2　b.Save(m,ImageFormat.Png)でクラッシュ

    using(var i = Image.FromFile(path))
    using(var b = new Bitmap(i))
    using(var m = new MemoryStream()) {
        b.Save(m,ImageFormat.Png);
    }


    //正常に動作

    using(var i = Image.FromFile(path))
    using(var b = new Bitmap(i))
    using(var m = new MemoryStream()) {
        b.Save("./コピー.png");
    }
}