To rename a file (Image) from your Imaging database, just search for the Image or browse to its location. Once it's displayed, you can click its corresponding Action icon and select "Rename image":
You'll then see the below:
You can simply type your New Name over the Image's current name and then click "Rename". You'll then see the Image displaying its new name within your Library, along with a notification above the grid verifying the change you've just made.