To do this, just hit Home, then Customization, followed by navigating to the Branding tab. To start making a welcome screen, just scroll down to the Welcome Screen section.
You will be presented with a page where you can give the screen a title and description. You can also attach an image or a video from your existing Materials list using the Pick from Existing Content drop-down menu.
You can do this more than one time and greet your users with multiple slides in the Welcome Screen. To reorder the slides, simply drag the Move Icon to where you want the slide to go (first slide is the top one). You can preview the Welcome Screen by clicking the Preview All button.
This is what the welcome screen could look like.