This example shows how save a bitmap in a memory stream and how to safe pass it to another thread without use locking techniques.

ThreadMsg.pas

procedure YourClass.SaveBMP;
var
  ms:TMemoryStream;
begin
  //your code
  ms:=TMemoryStream.Create;
  yourBMP.SaveToStream(ms); //instead of save to file
  yourThreadInstance.SendBMPStream(ms);
end;

//class ClassOfYourThread should have a field to the current stream;
//in this example I'll call FCurStream;
procedure ClassOfYourThread.SendBMPStream(ms:TStream); 
var
  aux:TStream;
begin
  aux:=InterlockedExchange(FCurStream, ms);
  //if aux isn't nil, the thread doesn't have processed it yet and this 
  //procedure updated FCurStream with another stream instance,
  //so I should free the previous unprocessed stream.
  if assigned(aux) then
   aux.free;
end;

procedure ClassOfYourThread.Execute; //thread main loop
var
  aux:TStream;
begin
  while not terminated do begin
    //atomic assigns nil to FCurStream and return the old value 
    //of FCurStream to aux;
    aux:=InterlockedExchange(FCurStream, nil);
    if assigned(aux) then begin
      FTelegramBot.SendPictureStream(aux...); //don't remember the exact name
      aux.free;
    end;
  end;
end;

{

Considerations:

** if the producer is faster than consumer (thread), some pictures/stream will be lost. If you don't want lost any picture/stream, replace the InterlockedExchange by TThreadList instance.

}