on iOS you can use a custom splash screen as described here in the docs.
For Android, you can use the following approach (which also works for iOS and other platforms of course):
1.) First only load a custom splash image
2.) With a Loader element, then load the real game dynamically after showing the initial splash image.
You can have a look how we implemented this functionality in our demo games, for example in the recent StackWithFriends demo introduced with daily build 1.5.5.