I would check your points. i see that b and d are swapped, but is there sufficient distances for you to see it with the image behind? are you sure that there isn't just a 1~2 pixel line somewhere or that start and end points are the same. you could be drawing a dot, or too small of a line to notice anything.
x = rand(500,500,3);
a=[0 500];
b = [0 500];
c = [500 0];
d = [250 0];
imshow(x),colormap jet
hold on
plot(a,b,'blue',c,d,'green')
works and draws the image and lines.
