See if this is what you want:
% Read in image.
grayImage = imread('pout.tif');
subplot(2, 1, 1);
imshow(grayImage);
% Get histogram.
[counts, grayLevels] = imhist(grayImage);
subplot(2, 1, 2);
bar(grayLevels, counts, 1);
grid on;
title('Histogram of image', 'FontSize', 20);
% Get the min and max gray levels directly from the image.
minGLImage = min(grayImage(:))
maxGLImage = max(grayImage(:))
% Get the min and max gray levels from the histogram (instead of directly from the image).
minGL = grayLevels(find(counts, 1, 'first'))
maxGL = grayLevels(find(counts, 1, 'last'))
% Should be the same as from the image.
% Get the rows and columns where these occur in the image.
[minRows, minColumns] = find(grayImage == minGL);
[maxRows, maxColumns] = find(grayImage == maxGL);
% Plot these locations over the image in the graphical overlay.
subplot(2, 1, 1);
hold on; % Don't blow away image.
plot(minColumns, minRows, 'b.', 'MarkerSize', 25);
plot(maxColumns, maxRows, 'r.', 'MarkerSize', 25);
title('Red = max. Blue = min.', 'FontSize', 20);
