data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAC4AAAA9CAMAAADlJaswAAAAe1BMVE.....

try
{
    byte[] streamBase = Convert.FromBase64String(base64);
}
catch(Exception ex)
{
    Console.WriteLine(ex.Message);
}

输入的不是有效的 Base-64 字符串，因为它包含非 Base-64 字符、两个以上的填充字符，或者填充字符间包含非法字符。

//base64 = base64.Replace("data:image/png;base64,", "").Replace("data:image/jgp;base64,", "").Replace("data:image/jpg;base64,", "").Replace("data:image/jpeg;base64,", "");//将base64头部信息替换
string imagebase64 = base64.Substring(base64.IndexOf(",") + 1);

string imagebase64 = base64.Substring(base64.IndexOf(",") + 1);
byte[] streamBase = Convert.FromBase64String(imagebase64);
BitmapImage bi = new BitmapImage();
bi.BeginInit();
bi.StreamSource = new MemoryStream(streamBase);
bi.EndInit();
Image.Source = bi;