Lo he resuelto... resulta que habia que castear como pictureBox... queda asi
Código ASP:
Ver originalFor Each TabX In TabAgo.Controls
For Each pic In TabX.Controls
If (TypeOf pic Is PictureBox And CType(Picturebox,pic).name.contains("PB")) Then
AddHandler pic.MouseDown, AddressOf MDown
End If
Next
Next