From BlenderWiki

[edit] Editing the Splash Screen

Editing the Splash Screen is a fairly straight forward process, suitable for your first blender programing attempt.

[edit] Finding the appropriate files in the source files

[edit] release/datafiles/splash.jpg

This is the location of the current splash screen being used by blender. First you have to create your new splash screen. Do this by editing the current splash screen in your favorite image editing software ie Photoshop, Gimp even Microsoft's Paint program will do.

Once you have created your splash screen save it as splash.jpg

[edit] release/datafiles/datatoc.c

Find this file in the same location and compile it using any compiler you have. datatoc.c is a small program which converts the JPG image to text format suitable for blender to interpret.

I compiled it with the following commmand

gcc -o datatoc datatoc.c

once compiled you will have a program called datatoc you then use this program to convert your splash.jpg to a splash.jpg.c file by typing the following at the command prompt.

datatoc splash.jpg

giving you a splash.jpg.c file

copy this file to

[edit] source/blender/src

replacing the original

Now compile blender as you would normally.

All done you now have a new splash screen.