The Hand tool is designed to scroll the displayed area of the image in the window if the whole image doesn't fit into the window at the chosen scale. To move the image you should press this button, put the cursor onto the image, press the left button and, keeping it pressed, move the image in the window. Press Alt+H to quickly access this tool.
Double-clicking on the tool's icon on the Toolbar makes the image fit the Image Window (Fit to View).
Even if another tool is active, you can press the spacebar on the keyboard and drag the image with the left mouse button to scroll the image in the Image Window.