When you opened the ScreenMan control panel, you probably were asked whether or not to create a new StartupScreen file. I am glad you told ScreenMan to do so. ScreenMan relies on having this file (a file called “StartupScreen” in your system folder) handy, so it creates one (after asking you for your permission). If there already is a StartupScreen file you got from another source, it is renamed to “Old StartupScreen”.

There is no problem in doing so. The preferences are marginal in size and the default picture (titled “Welcome to ScreenMan”) can easily be done away with as soon as you have any other startup screens enabled.

After these first time actions you are presented the usual user interface.
Every time you open ScreenMan, it will automatically select the picture which will come up at next startup.