First step is to create a PNG that matches your screens resolution as good as possible. After you have that you login to your Raspberry Pi and install a tool called fbi:

aptitude install fbi

After that move the splashscreen png to /etc/splashscreen.png, after you have that, create this init script with your editor in /etc/init.d/asplashscreen

#! /bin/sh
### BEGIN INIT INFO
# Provides:          asplashscreen
# Required-Start:
# Required-Stop:
# Should-Start:      
# Default-Start:     S
# Default-Stop:
# Short-Description: Show custom splashscreen
# Description:       Show custom splashscreen
### END INIT INFO

do_start () {
    /usr/bin/fbi -T 1 -noverbose -a /etc/splash.png    
    exit 0
}
case "$1" in
  start|"")
    do_start
    ;;
  restart|reload|force-reload)
    echo "Error: argument '$1' not supported" >&2
    exit 3
    ;;
  stop)
    # No-op
    ;;
  status)
    exit 0
    ;;
  *)
    echo "Usage: asplashscreen [start|stop]" >&2
    exit 3
    ;;
esac
:

After that we activate the init script and reboot the box:

chmod a+x /etc/init.d/asplashscreen
insserv /etc/init.d/asplashscreen
reboot

That should do the trick, enjoy!