You can't alter the splash screen which shows when the project is loading but you can mimic your own splash screen by adding a timer to the first screen of your app.
To add this:
- Open your project > hover over the first image > click on Edit
- In the top left-hand corner, click on Timer
- Add the amount of seconds you want the image to display for
- Select a destination screen. This will be the next screen which loads after the timer screen has transitioned.