Use the _mousedown event. It uses the Button parameter.
Just use simple if statement or case select to get the button clicked and then run whatever code you want.
eg:
Code:
Private Sub Image_MouseDown(Button As Integer, Shift As Integer, X As Single, Y As Single)
Select Case Button
Case 1 '1 is left button
MsgBox "Left Button Click"
Case 2 '2 is right button
Button = 0
End Select
End Sub