试一试
var Image1,Image2:TImage;
begin
Image1:=TImage.Create(nil);
Image2:=Timage.Create(nil);
Image1.loadformfile('...');
Image2.Height:=IMage1.Height/2;
Image2.Width:=Image1.Width/2;
Image2.Canvas.StrentchDraw(0,0,IMage2.Width,Image2.Height,Image1.Graphic);
Image2.Save('...');
Image1.free;
Image2.free;
end;