GetEncoderClsid('image/jpeg', encoderClsid);
..
Input_for_png := TGPBitmap.Create(FromName);
out_height :=Input_for_png.GetHeight;
out_width :=Input_for_png.GetWidth;
Output_for_png :=TGPBitmap.Create(out_width,out_height{,PixelFormat32bppARGB});
g := TGPGraphics.Create(Output_for_png );

g.DrawImage(Input_for_png, MakeRect(0, 0, out_width, out_height),   0, 0,out_width, out_height, UnitPixel);

result:=Output_for_png.Save(ToName, encoderClsid,@encoderParameters)=OK;

g: TGPGraphics;
SolidBrush: TGPSolidBrush;
in_img, out_img: TGPImage;

..
in_img := TGPImage.Create(FromName); // <-png с альфой
out_img :=TGPBitmap.Create(width,out_height);
g := TGPGraphics.Create(out_img);

 SolidBrush := TGPSolidBrush.Create(MakeColor(255,255, 255, 255));
 g.FillRectangle(solidBrush, 0, 0, out_width, out_height);
 SolidBrush.Free;

// тут рисуем из файла
//и сохраняем out_img