参考：https://bbs.csdn.net/topics/392161180?depth_1-utm_source=distribute.pc_relevant_download.none-task-discussion_topic-BlogCommendFromBaidu-5.nonecase&utm_medium=distribute.pc_relevant_download.none-task-discussion_topic-BlogCommendFromBaidu-5.nonecase

比较早的Delphi不支持pngimage单元，需要另外附加一个。

uses 
     pngimage,jpeg;
var
     jp:TJPEGImage;
     png: TPngImage;
     tmp:TBitmap;
begin
     png:=TPngImage.Create;
     jp:=TJPEGImage.Create;
     tmp:=Tbitmap.Create;
     try
          png.LoadFromFile('d:\a.png');
          jp.LoadFromFile('d:\b.jpg');
          tmp.Assign(jp);
          tmp.Canvas.Draw(0,0, png);
          jp.Assign(tmp);
          Image1.Picture.Assign(jp);
          Image1.Picture.SaveToFile('d:\c.jpg');
     finally
          png.Free;
          jp.Free;
          tmp.Free;
     end;
end;