Read the Base64 data into an Image class instance viaq a stream
Save the Image as a JPG into a Stream, and convert that to Base64.
private string FromBase64PNGToBase64JPG(string base64PNG)
{
byte[] bytes = Convert.FromBase64String(base64PNG);
using (MemoryStream msIn = new MemoryStream(bytes))
{
using (Image pic = Image.FromStream(msIn))
{
using (MemoryStream msOut = new MemoryStream())
{
pic.Save(msOut, System.Drawing.Imaging.ImageFormat.Jpeg);
return Convert.ToBase64String(msOut.ToArray());
}
}
}
}