This will enlarge the image, the window stays at the same size and displays a section from the centre of the image. While in this state, where the size of the image is greater than the size of the window, the image can be moved around to display a different section. This is done by holding the left button on the mouse down and moving the mouse, which will drag the image with it.
Zoom in can also be done using the mouse wheel, moving the wheel away from the user, or by pressing the Up key
An alternative form of zoom in can be obtained by holding the Ctrl key down while holding the left mouse button down and moving the mouse across the image. This will display a white rectangle to show the area marked out. When the mouse button is released the image will be zoomed so that the marked out area will be displayed in the window, with possibly a small adjustment to maintain the correct aspect ratio.