Delphi - how to get a list of all files of directory

if you use delphi 2010 then you can use tdirectory.getfiles first add ioutils.pas to uses clause then write the following line of code in the event handler(in addition to code you already have in that event handler)

uses IOUtils;

 var
    path : string;
begin
    for Path in TDirectory.GetFiles(OpenPictureDialog1.filename)  do
        Listbox1.Items.Add(Path);{assuming OpenPictureDialog1 is the name you gave to your OpenPictureDialog control}
end;

@Himadri, the primary objective of the OpenPictureDialog is not select an directory, anyway if you are using this dialog with another purpose you can try this code.

Var
  Path    : String;
  SR      : TSearchRec;
  DirList : TStrings;
begin
  if OpenPictureDialog1.Execute then
  begin
    Path:=ExtractFileDir(OpenPictureDialog1.FileName); //Get the path of the selected file
    DirList:=TStringList.Create;
    try
          if FindFirst(Path + '*.*', faArchive, SR) = 0 then
          begin
            repeat
                DirList.Add(SR.Name); //Fill the list
            until FindNext(SR) <> 0;
            FindClose(SR);
          end;

     //do your stuff

    finally
     DirList.Free;
    end;
  end;

end;