You can add all sorts of media to your pages in Flux just by dragging it to the page or directly to a folder in your project. This may be directly from iPhoto or Aperture or you can use the built in Media Viewer to locate your images, audio and movies.
When Media Viewer is launched it loads thumbnails of the images and movies on your Mac. If you edit any media or add to a library you can update the Media Viewer by right clicking the library or folder name and selecting Reload.