You can zoom in and out to show more or less of a picture on the screen at once. This is useful if you want to see a big picture in its entirety, or if you want to inspect the fine details of a picture.
You can zoom using the mouse wheel. Scroll up to zoom in or scroll down to zoom out.
You can also zoom using the buttons in the header bar. To zoom in, click the Enlarge the image icon, and to zoom out click the Shrink the image icon. These look like "plus" and "minus" signs.
Alternatively, use the keyboard shortcuts Ctrl++ or Ctrl+-.
To view the picture in its original, unscaled size, choose the 100% zoom level from the dropdown in the header bar. Alternatively, use the keyboard shortcuts Ctrl+0 or 1.
Zoom to best-fit the window
You can choose to zoom in so that the picture fits fully in the window, even if you resize it. Enable Best fit in the dropdown in the header bar to do this.