关键代码如下：

var
Form1: TForm1;
x,y:Integer;
implementation

{$R *.dfm}

procedure TForm1.TrackBar1Change(Sender: TObject);
begin
if TrackBar1.Position=0 then
begin
image1.Width:=x;
image1.Height:=y;
Exit;
end;
image1.Width:=Ceil(x*( TrackBar1.Position));
image1.Height:=Ceil(y*( TrackBar1.Position));
image1.Top:=Ceil(1/2*(Self.Height-image1.Height));
Image1.Left:=Ceil(1/2*(Self.Width-image1.Width));
end;

procedure TForm1.FormShow(Sender: TObject);
begin
x:=image1.Width;
y:=image1.Height;
image1.Top:=Ceil(1/2*(Self.Height-image1.Height));
Image1.Left:=Ceil(1/2*(Self.Width-image1.Width));
end;

end.

需要引用Math单元..