MySQL BLOB data is gradually lost?

In the mysql table, MyISAMenter the Imagetype column mediumbloband save the captured image. I have interesting and problematic images. Some of the images are data gradually losing.

Field          type  
--------------------------
image         mediumblob

my.ini max allow packet size max_allowed_packet = 8M

This is problem

When the application C#retrieves data from the server every time these images lose data gradually and randomly. I got 10-12bad images like this in 100000+image data.

What could be causing this behavior? Anyone have an idea / solution on how to fix / avoid this problem.

Update 1:
Reading Bytes of a PictureBox Form

MemoryStream ms = new MemoryStream();
byte[] ret = null;

try
{
     picturebox.Image.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
     byte[] Data = new byte[ms.Length];
     ms.Read(Data, 0, (int)ms.Length);
     ret = byteData;
     ms.Close();
 }         

blob.

byte[] Data = (byte[])reader["Image"];