Use find(), not max():
v = randi(10, 5, 30)
[maxValue, indexes] = max(v(:))
[rows, columns] = find(v == max(v(:)))
So, find() works. The horrendous help for max says:
[M,I] = max(_) finds the indices of the maximum values of A and returns them in output vector I, using any of the input arguments in the previous syntaxes. If the maximum value occurs more than once, then max returns the index corresponding to the first occurrence.
So what the first sentence promised, the second sentence takes away. It does not return all maxima locations, only the first one per column.
Alternatively, you can also get the max locations as a binary image
binaryImage = grayImage == max(grayImage(:));