When you open any image in its own tab, if it will not all fit the image is auto-resized to your tab. If you click on the image, it will zoom to full size (with the spot you clicked on centered, if possible). Once it is at full size, zoom and scrolling will work - while it is auto-resized trying to zoom still leaves it resized to your tab and of course you can't scroll something that already fits completely inside a tab.
