An "Image Centered" slide is exactly as it sounds, an image that is not changed in width or height in any way and is centered in the panel. If the image is smaller than the panel width or height, then the "Background Color" should be set to provide the appropriate color to go with the image.
You can click on the browse button to open a file listing to locate the file. Display5 supports GIF, JPG, IMG, BMP, TIF and many other image formats. You can also add a transition to the image so that it loads with an effect. Specify the width and height of the image as well as any background color that you would like to add. You can also re-use an image from the show by selecting the library images link on the panel.