The viewer contains functions that allow you to flip images in a frame or thumbnail images horizontally or vertically, as well as rotate it to the right or left. Click to select the image you want to flip or rotate, and then apply the respective function from the Orientation tools group.
Flip and Rotate Keyboard shortcuts:
Ctrl-H - flip horizontally
Ctrl-U - flip vertically
Ctrl-L - rotate left
Ctrl-R - rotate right.
To reset the image orientation, use
the reset orientation 
icon.
This function is available in Commands column the Toolbar Manager and
must be added to a group.