Option Explicit
' Crea la región
Private Declare Function CreateRoundRectRgn Lib "gdi32" ( _
    ByVal X1 As Long, _
    ByVal Y1 As Long, _
    ByVal X2 As Long, _
    ByVal Y2 As Long, _
    ByVal X3 As Long, _
    ByVal Y3 As Long) As Long

'Establece la región
Private Declare Function SetWindowRgn Lib "user32" ( _
    ByVal hwnd As Long, _
    ByVal hRgn As Long, _
    ByVal bRedraw As Boolean) As Long

Private Sub Form_Load()
  Call Redondear_objetivo(Picture1, 15)
  Picture1.BackColor = vbRed
End Sub

Private Sub Redondear_objetivo(objetivo As PictureBox, Radio As Long)

Dim Region As Long
Dim ret As Long
Dim Ancho As Long
Dim Alto As Long

'Obtenemos el ancho y alto de la region del Form
Ancho = objetivo.Width / Screen.TwipsPerPixelX
Alto = objetivo.Height / Screen.TwipsPerPixelY

'Le pasamos el ancho alto del formualrio y el valor de _
 redondeo es decir el radio
Region = CreateRoundRectRgn(0, 0, Ancho, Alto, Radio, Radio)

' Aplica la región al formulario
ret = SetWindowRgn(objetivo.hwnd, Region, True)

End Sub