testimage=imread('pout.tif');
imshow(testimage)
H=imrect(gca);
pos=wait(H);
close all
testimage(pos(1,2):pos(1,2)+pos(1,4),pos(1,1):pos(1,1)+pos(1,3))=0;
imshow(testimage);
run this, it will let you create a draggable square, select the part of the image that you want to make black and then double click on the square, it will save your updated image to testimage. you can also set the 0 to something like mean2(testimage) so it's not completely black.
