unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, PNGImage;

type
  TForm1 = class(TForm)
    PaintBox1: TPaintBox;
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure PaintBox1Paint(Sender: TObject);
  private
    FBitmap: TBitmap;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
var
  PNGImage: TPNGImage;
begin
  FBitmap := TBitmap.Create;
  PNGImage := TPNGImage.Create;
  try
    PNGImage.LoadFromFile('C:\SomeFile.png');
    FBitmap.Assign(PNGImage);
  finally
    PNGImage.Free;
  end;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  FBitmap.Free;
end;

procedure TForm1.PaintBox1Paint(Sender: TObject);
begin
  // 128 in the fourth parameter is an example opacity value; it can be the value
  // in the range of 0 to 255, where 255 is fully opaque rendering
  PaintBox1.Canvas.Draw(0, 0, FBitmap, 128);
end;

end.