You cannot get the image name that way, so you need to set a value that you can read from the command button whenever the image changes. You could use a hidden check box or a hidden label or use the tag property of the userform e.g.
Code:
Private Sub UserForm_Initialize()
Label1.Picture = LoadPicture("D:\My Pictures\Image_1.bmp")
Label2.Caption = "Image1"
Label2.Visible = False
'Tag = "Image1"
End Sub
Private Sub CommandButton1_Click()
If Label2.Caption = "Image1" Then
'If Tag = "Image1" Then
Label1.Picture = LoadPicture("D:\My Pictures\Image_2.bmp")
Label2.Caption = "Image2"
'Tag = "Image2"
Else
Label1.Picture = LoadPicture("D:\My Pictures\Image_1.bmp")
Label2.Caption = "Image1"
'Tag = "Image1"
End If
End Sub