Just draw to your images canvas.
eg. something like.. btw. I'm coding from memory, so might be some syntax errors etc.
begin
with image1.picture.bitmap.canvas do
begin
brush.style := bsNone;
pen.Style := psDot;
pen.Color := clRed;
rectangle(0,0,image1.picture.bitmap.width,image1.picture.bitmap.height);
end;