By default Opus displays the Images toolbar whenever a file display is set to Thumbnails mode (you can change this in Preferences).

Rotate Left:
Rotates selected image files left by 90 degrees.
Rotate Right:
Rotates selected image files right by 90 degrees.
Convert: Clicking
the main button opens the Image Conversion dialog for selected images.
The drop-down menu contains command that let you quickly convert selected
images to a particular format (JPG, PNG, BMP or GIF).
Show: Displays selected
image files in the standalone
image viewer.
Set Wallpaper:
Displays a drop-down menu that lets you set the selected image file as your
desktop wallpaper.
Sort by name:
Sorts the folder alphabetically (by name).
Sort by date:
Sorts the folder by last modified date.
Sort by date
taken: Sorts the folder by the "date taken" field. Any non-image
files or those without a date taken field will be grouped at the bottom. These
three sort buttons are provided to make it easy to quickly select
commonly desired sorting options (because in Thumbnails mode the file display
header isn't displayed, which makes it harder to change the sort order).