Use your file manager, find the photo, long press on it to select it, go to the menu, choose "copy", then in file browser, choose where you want to copy the file to, then paste it there. After you verify the photo has copied correctly, then you can delete the original. Never use the move function as phones will delete the original before you can verify if the file moved correctly... by then, if the transfer didn't go right, you'll lose your photo. Also, depending on your phone, you may be able to do this through your gallery app menu.
If your phone doesn't have a file browser, you can install
ES File Explorer from playstore or search for an easier one to use.