An image is specified (like any other matrix) as MxNxP, where M is the number of rows, N is the number of columns, and P=1 or 3 if the image is indexed (P=1) or RGB (P=3). The horizontal coordinate (analgous to the X coordinate with data) would thus correspond to the number of columns. That is, X is the vertical position coordinate of the position, and Y is the horizontal position.
You should also take note of in which directions X and Y are increasing. Note that the upper left corner of the image is at (1,1), while the lower right corner is (M,N). For more information, see "axis ij" in the Help for "axis", i.e.
doc axis
