GetRValue调用出错，有的BMP图片编译运行显示‘Range Check Error'?(100分)

var R;Byte;
begin
Image1.Picture.LoadFromFile(OpenPictureDialog1.FileName);
for i:=1 to Image1.Width-1 then
for j:=1 to Image1.Height-1 then
R:=GetRValue(Image1.Canvas.Pixels[i,j]);
end;