for x := 0 to Image.Width do
for y := 0 to Image.Height do
begin
Dist := Sqrt((x * x) + (y * y));
if Dist < 50 then
Image.Pixels[x, y] := clBlack;
end;

This will draw a 1/4 circle in the left-top corner with a radius of 50. I did this from my naked mind, I didn't try the code, but I think you should be able to make something out of it.