Updating data with image

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VB

SQL-Server-2008R2

This code works when Updating data with image. However, The problem is when i edit the record that is existing and i do not change the image. The `picturebox` becomes null.

cmd.Parameters.Add(New SqlParameter("@StudPic", SqlDbType.Image)) 
If (Not String.IsNullOrEmpty(Me.Name) AndAlso System.IO.File.Exists(a.FileName)) Then 
cmd.Parameters("@StudPic").Value = System.IO.File.ReadAllBytes(a.FileName) 
Else 
cmd.Parameters("@StudPic").Value = DBNull.Value 
End If 

What i have tried is this

cmd.Parameters.Add(New SqlParameter("@StudPic", SqlDbType.Image)) 
If (Not String.IsNullOrEmpty(Me.Name) AndAlso System.IO.File.Exists(a.FileName)) Then 
cmd.Parameters("@StudPic").Value = System.IO.File.ReadAllBytes(a.FileName) 
Else 
cmd.Parameters("@StudPic").Value = System.IO.File.ReadAllBytes(a.FileName) 
End If 

But it throws me an error. Empty path is not legal with this line

cmd.Parameters("@StudPic").Value = System.IO.File.ReadAllBytes(a.FileName)

` . Can anyone help me to fix this. thanks in advance