Use the Import Pictures button to add your pictures to your storyboard at the bottom of the page. The slider (highlighted green below) will let you choose the size of the thumbnails you can see. Drag/Select to choose them all or Ctrl/Click to select individual slides.
You can always come back to this page if you decide you need to Import more, or different, images.
Use the tools at end of the storyboard to move or delete pictures, or drag images around into the right order for your story. Don't forget to add extra copies of pictures if you wish to use paths or add effects.