Cover a check box in the image to require a code for its selection - inno-setup

Question

I'm trying to create something like this:
I want the check box to be hidden behind the lock image, and clicking on it will open a form requesting code entry, and if the code is correct the image will be removed and the check box will be available.
Here's what I tried:
[files]
Source: "Lock.bmp"; Flags: dontcopy
{...}
[code]
var
CustomFuncPage: TWizardPage;
CBAdvFunctions: TNewCheckBox;
Locked: TBitmapImage;
procedure OnClickLock(Sender: TObject);
var
PassForm: TSetupForm;
Edit: TNewEdit;
OKButton, CancelButton: TNewButton;
begin
PassForm := CreateCustomForm();
PassForm.ClientWidth := ScaleX(256);
PassForm.ClientHeight := ScaleY(128);
PassForm.Caption := 'Password required';
Edit := TNewEdit.Create(PassForm);
Edit.Top := ScaleY(10);
Edit.Left := ScaleX(10);
Edit.Width := PassForm.ClientWidth - ScaleX(20);
Edit.Height := ScaleY(23);
Edit.Anchors := [akLeft, akTop, akRight];
Edit.Parent := PassForm;
OKButton := TNewButton.Create(PassForm);
OKButton.Parent := PassForm;
OKButton.Caption := 'OK';
OKButton.Left := PassForm.ClientWidth - ScaleX(166);
OKButton.Top := PassForm.ClientHeight - ScaleY(33);
OKButton.Height := ScaleY(23);
OKButton.Anchors := [akRight, akBottom]
OKButton.ModalResult := mrOk;
OKButton.Default := True;
CancelButton := TNewButton.Create(PassForm);
CancelButton.Parent := PassForm;
CancelButton.Caption := 'Cancel';
CancelButton.Left := PassForm.ClientWidth - ScaleX(85);
CancelButton.Top := PassForm.ClientHeight - ScaleY(33);
CancelButton.Height := ScaleY(23);
CancelButton.Anchors := [akRight, akBottom]
CancelButton.ModalResult := mrCancel;
CancelButton.Cancel := True;
PassForm.ActiveControl := Edit;
if PassForm.ShowModal() = mrOk then
begin
if Edit.Text = '1234' then
begin
Locked.Free;
PassForm.Free;
end else
begin
MsgBox('Password incorrect', mbError, MB_OK);
PassForm.Free;
end;
end else
begin
PassForm.Free;
end;
end;
Procedure InitializeWizard;
begin
CustomFuncPage := CreateCustomPage(wpSelectProgramGroup, {...}, {...});
CBAdvFunctions := TNewCheckBox.Create(CustomFuncPage);
CBAdvFunctions.Parent := CustomFuncPage.Surface;
CBAdvFunctions.Top := 0;
CBAdvFunctions.Left := 0;
CBAdvFunctions.Caption := 'Advanced Functions'
CBAdvFunctions.Width := ScaleX(130);
CBAdvFunctions.Cursor := 1;
CBAdvFunctions.SendToBack;
ExtractTemporaryFile('Lock.bmp');
Locked := TBitmapImage.Create(CustomFuncPage);
Locked.Parent := CustomFuncPage.Surface;
Locked.Bitmap.LoadFromFile(ExpandConstant('{tmp}\Lock.bmp'));
Locked.Bitmap.AlphaFormat := afPremultiplied;
Locked.SetBounds(0, 0, CBAdvFunctions.Width, CBAdvFunctions.Height);
Locked.Stretch := True;
Locked.BringToFront;
Locked.OnClick := #OnClickLock;
{...}
And this is the result:
This is the image I used:
I tried a few ways, but was unable to get the image to cover the checkbox.
Maybe someone has an idea?
By the way, maybe someone has an explanation for why the background of the image is not transparent?