In the FSX subdirectory Uires there is a file dlgsplash.bmp which is your splash screen. Make a backup of the original, name your bmp file which you want to use as your new splash screen dlgsplash.bmp and replace the original. You may need to fool around with the dimensions to suit your needs.
He left out two very important details: The image dimensions must be 800x602 and the filetype as bmp.
8-)
Not only that, but the file size can't be more than about 940 kb. I tried one that was the right dimensions but was rather large in file size and FSX opened without a splash screen.
If God intended aircraft engines to have horizontally opposed engines, Pratt and Whitney would have made them that way.