This specifies the mode in which KPager paints the windows, this are the available modes:
Plain - Draw windows as other pagers (just rectangles)
Icon - Draw windows as rectangles and the icon of the application on it, so that you can easily recognize the application.
Pixmap - Grab the window from the screen and draw a resized screenshot of it. Note that this is an extremely nice option but at the same time, it is resource hogging (in fact, I would recommend a 200 Mhz machine with a good graphic card to use it)