When zoomed in, you can scroll an image to see the parts of it that are out of view.
Do one of the following:
Hold down Space on your keyboard and drag to scroll the image.
When you do this, the cursor will change from a pointer to a hand. This lets you know the Hand tool is active. With certain tools, like the Color Adjustments or Effects, the Hand tool is automatically activated.
Scroll by sliding two fingers on your Multi-Touch trackpad, just like you would in apps like Safari.