The default settings depend entirely on your pictures. For instance, if you have a picture in portrait layout and a widescreen monitor, there will be significant amounts of black space on either side so that it is not horribly distorted by the stretching process. Also, if the image is low quality and, by default, is displayed as a small image, it will not be blown up in order to prevent the picture from becoming pixelated, another form of distortion. You can override those settings if you want and accept the results, but otherwise your options are leave it as is or choose only images that are designed to properly fit your screen.
I am looking for a screensaver that will display my pictures on the monitor. I want the pictures to basically take up the whole screen. I have found a couple screensavers where the pictures come up fairly small on the screen, but I really would rather they filled the whole screen if possible. If somebody could offer me some ideas I would really appreciate it.