Each slide can have its own background image. Click the 
button at the Side Toolbar to choose a background image for the current slide.
Check the Have background image checkbox to enable the background image. Then, click the Browse button to choose an image file.
Click the OK button to apply the changes.
You will notice that the slide background is now changed to the selected image file. Flash Demo Builder always resizes the image to fit the presentation dimension while preserving the image aspect ratio.
Note that background image is a special object and will never be appeared in the Timeline menu. However you can still double click the empty area/background image within the slide to bring the Slide Background Properties window.
Flash Demo Builder allows you to add more images into a slide by adding an Image object. An Image object is resizeable and rotatable. It is visible in the Timeline menu.
Next - Changing Project Settings