Thread Subject: plotting over images - different output resolution!

Hi all, I'm facing a problem with image resolution. I'm dealing with 780x250 bmp images and an array of points. I want to print these points over the pictures and I do it in this way:

for count=1:s
filename=strcat(imgName,num2str(count+refImg-1),'.BMP');
I=imshow(filename);
img=imread(filename);
height = length(img(:,1));
hold on;
plot(x,height-y,'r');
plot(x,height-y,'g.');
plot(x(1:8:end),height-y(1:8:end),'b*');
img=['output/' inputName];
outImg=sprintf([img 'm%d.jpg'],count+refImg-1);
end

it works but unfortunately the output is in a different resolution (1200x900). The images contain my output in the center but are surrounded by a white border. How can i delete this and get my output as 780x250 (jpg or bmp) image?
I guess i should set a parameter in sprintf or use a different command! Suggestions?
Thank you!

Mario