This is an IE only setting.
It makes it possible to define a list of images to be downloaded and cached before enabling FCKeditor to load.
This is useful for skins that use CSS images. IE has a well known bug that prevents background images from being taken from the cache when several requests for that same image are made together. It repeats the requests to the server instead. By caching then with this setting, the image is downloaded only once, and then all successive requests are taken from the cache. Syntax:
FCKConfig.PreloadImages = [ FCKConfig.SkinPath + 'images/toolbar.start.gif', FCKConfig.SkinPath + 'images/toolbar.buttonarrow.gif' ] ;