There's the integrated Viewer Pane, which is embedded within a Lister:

There's also a standalone (floating) image viewer.

The main difference is the Viewer Pane is tied to a Lister whereas the standalone viewer acts almost like a separate utility. The standalone viewer also has basic image editing functions which the viewer pane does not.

You can access the standalone viewer in several ways:

• If the Use internal picture viewer option on the File Operations / Double-click on Files page in Preferences is turned on, double-clicking on a recognized image file will open it in the standalone viewer.

• The Slideshow button on the default toolbar will launch a slideshow of all images in the current folder, using the standalone viewer. The speed of the slideshow is controlled from the Slideshow page in Preferences.

• The internal Show command will display selected files in the standalone viewer. This command is available on the drop-down menu attached to the Slideshow button.
  

• From outside of Opus, you can use the d8viewer.exe or dopusrt.exe /show commands to open files with the Opus viewer.

There are a number of options that control the appearance and behaviour of the standalone viewer. These can be found in the Viewer category in Preferences. By default, the viewer will:

• Auto-size to fit every picture - as you step through images, the window will resize if needed to display the picture.
• Open centered on the current monitor.
• Display a frame around the picture (as in the above screenshot).
• Show or hide the scrollbars while viewing images.
• Automatically build a list of all other pictures in the folder, when opened via a double-click on an image file (with an additional option for the list to wrap-around when you reach the start or end).
• Automatically rotate images to compensate for the EXIF orientation tag, saved by most digital cameras.

These options can all be changed from Preferences.

More:
Viewer Mouse, Keys and Toolbar
Image Marking