You'd have to make it in an external program which afterwards run your application. If you're fine with having your splash inside the application you could just create a GUI-texture or put the texture onto a cube which is shown at start.

1. create a new scene
2. add new gui texture(the splash screen which you want)

3. create a new script and place it on the main camera

   code for the script

   function Start() { yield WaitForSeconds(the time which you want the splash screen to appear); Application.LoadLevel(the next level which you want to load); }

4. go to file-> buildsettings : drag the scene with the splash image to the top so that it gets the build number 0

Ok Would you happen to know any program that I can use to make it Thanks