Click once and leave the mouse cursor on the button to zoom the image closer. Moving the mouse cursor off the button and away from the image stops the zoom motion. Click on the button again to zoom in more.
Click once and leave the mouse cursor on the button to zoom out. Moving the mouse cursor off the button and away from the image stops the zoom out motion. Click on the button again to zoom out more.
You can also move the image by clicking on it and holding down the mouse button, dragging the mouse left or right, and up and down if you have zoomed into the image.