Remove the Calculate-event the try this one. 
Private Sub Worksheet_Change(ByVal Target As Range)
    Const numpix = 37.7952755905511 'pixels per cm
    If Target.Address = "$P$3" Then
        Dim mypic As Picture
        Me.Pictures.Visible = False

        With Range("e2")
            For Each mypic In Me.Pictures
                If mypic.Name = .Text Then
                    mypic.Visible = True
                    mypic.Top = .Top
                    mypic.Left = .Left
                    Exit For
                End If
            Next mypic
        End With
        Dim wpicture As String, s As Double, y As Double
        With Sheet2
            wpicture = .Cells(Application.Match(Target.Value, .Columns(1), 0), 2)
        End With
        With Sheet1
            s = Round(.Shapes(wpicture).Height / numpix, 2)
            y = Round(.Shapes(wpicture).Width / numpix, 2)
        End With
    
        MsgBox "Picture dimensions are " & vbLf & vbLf & _
            "Height: " & s & " cm" & vbLf & vbLf & _
            "Width: " & y & " cm"
    End If
End Sub