Ciao Pietro,

codice:
Dim imm As Image = Image.FromFile(@"C:\miacartella\SenzaTag.jpg")
Dim tag = imm.PropertyItems.FirstOrDefault(Function(w) w.Id = &H131)
If tag Is Nothing Then
    Dim mType = GetType(PropertyItem)
    Dim ctor = mType.GetConstructor(BindingFlags.NonPublic Or BindingFlags.Instance Or BindingFlags.[Public], Nothing, New Type() {}, Nothing)
    tag = DirectCast(ctor.Invoke(Nothing), PropertyItem)
End If
Dim text As String = "Libreria" & Char.MinValue
tag.Id = &H131
tag.Len = text.Length
tag.Type = 2
tag.Value = Encoding.UTF8.GetBytes(text)
imm.SetPropertyItem(tag)
imm.Save("C:\miacartella\conTag.jpg")