From the main menu click on Photos > Galleries and locate the gallery you want to work with.
On that gallery click on Photos > Manage Categories then click on the name of the category you want to work with.
Select one or more images that you want to rotate then click the Actions box in the upper right.
Scroll down that list and click Rotate. Please note that you may need to scroll the list to get to that and other options.
Select your desired Photo Rotation and click Rotate.
You may need to refresh your browser window to see the rotated images.