In Pages, click on the Photo. The Image tab now appears and on that panel, there is File Info with a generic icon and filename. Click once on that icon and in one gesture, drag and drop it onto your Desktop. You now have a copy of your original image. That is for any photo that you just dragged and dropped into Pages.
If you used an Image Gallery, Click on the Gallery to select it. This will expose the Gallery tab on the right-side of Pages where the individual images in the Gallery are listed. You can single-click and drag these entries out over the Desktop and drop to retrieve them from Pages. The "Desktop" reference can be any folder in an open Finder Window too.