CreateEllipticRgn 楕円形のリージョンを作成
CombineRgn 既存の二つの領域を結合して新しい領域を作成
SetWindowRgn 指定の領域をウィンドウ領域として設定
RGN_OR(2) リージョン同士のOR結合

PictureBoxをコマンドボタン代わりにしてみました。

'================================================================
'= コマンドボタンの作成
'=    (CombineRgn2.bas)
'================================================================
#include "Windows.bi"

' 楕円形のリージョンを作成
Declare Function Api_CreateEllipticRgn& Lib "gdi32" Alias "CreateEllipticRgn" (ByVal nLeftRect&, ByVal nTopRect&, ByVal nRightRect&, ByVal nBottomRect&)

' 既存の二つの領域を結合して新しい領域を作成
Declare Function Api_CombineRgn& Lib "gdi32" Alias "CombineRgn" (ByVal hRgnDest&, ByVal hRgnSrc1&, ByVal hRgnSrc2&, ByVal nCombineMode&)

' 指定の領域をウィンドウ領域として設定
Declare Function Api_SetWindowRgn& Lib "user32" Alias "SetWindowRgn" (ByVal hWnd&, ByVal hRgn&, ByVal bRedraw&)

#define RGN_OR 2                        'リージョン同士のOR結合

Var Shared Bitmap As Object
BitmapObject Bitmap

Var Shared Picture1 As Object
Var Shared Text1 As Object

Picture1.Attach GetDlgItem("Picture1")
Text1.Attach GetDlgItem("Text1") : Text1.SetFontSize 14

'================================================================
'=
'================================================================
Declare Sub MainForm_Start edecl ()
Sub MainForm_Start()
    Var Rgn0 As Long
    Var Rgn1 As Long
    Var Rgn2 As Long
    Var Rgn3 As Long
    Var Ret As Long

    Bitmap.LoadFile "Mickey1.bmp"
    Picture1.StretchBitmap Bitmap, 0, 0, 80, 80
    Bitmap.DeleteObject

    Rgn0 = Api_CreateEllipticRgn(14, 20, 68, 75)      '顔中央部
    Rgn1 = Api_CreateEllipticRgn(3, 3, 30, 33)        '左耳
    Rgn2 = Api_CreateEllipticRgn(52, 2, 77, 33)       '右耳
    Rgn3 = Api_CreateEllipticRgn(26, 33, 53, 79)      '顎

    Ret = Api_CombineRgn(Rgn0, Rgn0, Rgn1, RGN_OR)    'Rgn0 と Rgn1 を合成 = Rgn0
    Ret = Api_CombineRgn(Rgn0, Rgn0, Rgn2, RGN_OR)    'Rgn0 と Rgn2 を合成 = Rgn0
    Ret = Api_CombineRgn(Rgn0, Rgn0, Rgn3, RGN_OR)    'Rgn0 と Rgn3 を合成 = Rgn0
    
    Ret = Api_SetWindowRgn(Picture1.GethWnd, Rgn0, True)
    Picture1.ShowWindow -1
End Sub

'================================================================
'=
'================================================================
Declare Sub Picture1_MouseDown edecl (ByVal Button As Integer, ByVal Shift As Integer, ByVal x As Single, ByVal y As Single)
Sub Picture1_MouseDown(ByVal Button As Integer, ByVal Shift As Integer, ByVal x As Single, ByVal y As Single)

    If Button = 1 Then
        Picture1.MoveWindow 9, 9
        Bitmap.LoadFile "Mickey2.bmp"
        Picture1.StretchBitmap Bitmap, 0, 0, 80, 80
        Bitmap.DeleteObject
    End If
End Sub

'================================================================
'=
'================================================================
Declare Sub Picture1_MouseUp edecl (ByVal Button As Integer, ByVal Shift As Integer, ByVal x As Single, ByVal y As Single)
Sub Picture1_MouseUp(ByVal Button As Integer, ByVal Shift As Integer, ByVal x As Single, ByVal y As Single)

    Picture1.MoveWindow 8, 8
    Bitmap.LoadFile "Mickey1.bmp"
    Picture1.StretchBitmap Bitmap, 0, 0, 80, 80
    Bitmap.DeleteObject

    Text1.SetWindowtext "ミッキーマウスボタンが押されました！"
    Wait 100
    Text1.SetWindowText ""
End Sub

'================================================================
'=
'================================================================
While 1
    WaitEvent
Wend
Stop
End