I’ve added pictures named the screensaver and saved the file to desktop but can’t see how to open it and make it work. If I go to system preferences I find the file greyed out on the select a folder screen.
iScreensaver Personal will create a .Saver file - simply double-click the .Saver file to install it.
However, sometimes after installing the screensaver, OS X doesn’t properly select it for use.
This is easy to fix: http://iscreensaver.com/help/notes/osx_find_screensaver/osx_find_screensaver.shtml