img = imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/562881/image.jpeg');
sz = size(img);
endx = min(sz(1), sz(2));
h(1) = image(img);
hold on
h(2) = plot([1 endx], [1 endx], 'r');
hold off
Or if you need the line "burned in" to the image:
img = imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/562881/image.jpeg');
sz = size(img);
endx = min(sz(1), sz(2));
nimg = insertShape(img, 'Line', [1 1 endx endx], 'Color', 'r');
h(1) = image(nimg);
The difference between these two is that in the first version, the red line is only visible in what is drawn, with it not being made part of the image array; h will end up as a vector containing the handle to the image display object that only knows about the gray scale image, and a handle to the line display object. In the second version, the red line is part of the array stored in nimg, suitable for further processing (or writing to a file), and h will be the handle of the image display object that only knows about displaying the array that inherently has the red line as part of the data.
