To be honest, I've never understood the value/need for the 'notext' versions of the splash screen. I've created them when I've made my own just in case there's some use of them that I'm not aware of, but as far as I've ever been able to tell, sphalsh24b.bmp is the intro, and splash.bmp gets used while the live sim GUI is loading (if you have that enabled), and if you don't, it's simply the background of the GUI.
For the record, the above is not what I'm planning for splash.bmp. I built it at 800x600 so enlarging it is not my preference. I'm toying with other ideas.