You have to build this with Blender.
It is located at /blender/release/datafiles/splash.png — (old svn link) (new git link).
Just replace1 this file with a similar one.
- 1Here is a link to the entry on the wiki on how to correctly change it.
- You can grab an old template here which should still work and size your image accordingly (src). The template is also in the same directory as
splash_template.xcf - BlenderCookie has a tutorial on building Blender in which the author changes the splash screen. (He uses mac but the majority of process can be done pretty much the same way for other platforms)
