Rename operation is only available for one selected image file.
To rename selected image file right click on the selection and choose the Rename file command from the pop-up menu. The Operations... toolbar button and the Files submenu of the Main Menu contain the same menu command. You will be prompted for a new file name. A file will remain in the same folder. To rename a file and save it into another folder use the Move selected files to... command from the pop-up menu.