Go to "Project" menu, select "Project settings…" which opens the "Project Settings" dialog. On the left panel select "Application" and inside "Boot Splash". On the right you will find the setting "Image", you can select the image you want to appear there.
Leaving it blank will result in the default image.
And yes, you can place a path that does not exist, and it will fail loading the image (instead you get a white screen), and the game still shows up. However, I suggest to place your own image instead.
I'll also suggest to use an image with transparency, and have black background. Why black? Well, if you export for Android and rotate the screen while the splash screen is up, it will show black bars while rotating. If the background is also black, the user won't notice.
As you know, the splash screen will stay up as long as it takes to load the start scene. This means that a start scene that loads faster translates to less splash screen time.
You may take this to your advantage: create a small scene that shows a picture that matches the splash screen (so the user does not notice the change), and loads your main scene in the background (see Background loading). That way you can have a progress bar, have animation, and fade out to reveal your main scene.