If your goal is to find the point of highest intensity, why are you using ginput to do it manually? Are you trying to find the highest intensity in a particular user-selected local region? If so, you should realize that you're not likely to have a very good time finding it if your image is scaled for display.
Assuming that's the actual root purpose, you can just define an ROI and find the max within that. For example:
inpict = imread('coins.png');
s = size(inpict);
imshow(inpict); hold on;
P = drawpolygon(); % select one of the coins with a polygon selection
maskedpict = inpict.*uint8(createMask(P));
[mx idx] = max(maskedpict(:)) % max value and linear index
[row col] = ind2sub(s(1:2),idx) % or row and column subs
delete(P);
plot(col,row,'bo') % show max location
Otherwise, you might want to clarify.
