TImageEnMView.OnCheckboxClick
 
Declaration
property OnCheckboxClick: TIECheckboxClickEvent;
Description
Occurs as a user clicks a checkbox so you can allow or prevent selection.
idx is the index of the clicked image.
bChecked specifies the new status of the image. You can override it, e.g. set it to false if the image cannot be checked
Note: Don't read 
Checked or 
CheckedCount in this event which will not yet be valid. Use 
OnCheckboxChange
procedure TForm1.ImageEnMView1CheckboxClick(Sender: TObject; idx: integer; var bChecked : Boolean);
begin
  // Only allow JPEG images to be checked
  if bChecked and (IEFileIsOfFormat(ImageEnMView1.ImageFilename[idx], ioJPEG) = False) then
  begin
    MessageBeep(MB_ICONEXCLAMATION);
    bChecked := False;
  end;
end;
See also
◼Checkboxes
◼CheckedCount