I wouldn't trust boot screen changes though, especially if your using a program to do so with a shareware function of sorts.
I've had one once which was supposed to be quite good, booted up fine, yet restarted the computer again after trying to change the old one back (I'm sure I did it right), and it crashed and wouldn't start up.
I needed to go into safemode, uninstall the program, and strangely...it worked...
I'm sure some are okay though...just not the one I tried.
I also find, if you cant make up your own designs using the prgrams, whats the point? The custom ones already on the sites dont usually go to your tastes I find.
I wonder if there are any that do let you create?
Thats the only way i think you'd be able to do it though without having expert knowledge of how windows uses the files.