You can click and drag the scroll-grabber button on the vertical scroll bar on the left side of an image window. Dragging the button up zooms the image in and dragging it down zooms it out. You can also click to select anywhere on the left-side scroll bar to jump to specific magnifications.
With the image window selected, you can also use your mouse wheel or trackpad to zoom in and out.
Click to select and hold to drag an image to move it around inside its document window.
You can also click and drag the highlighted area in the thumbnail image in the upper right of the image document window, and the image display will adjust accordingly.
You can also click to select any part of the thumbnail image to center the image display on that point of the image: