To add own splash screen you need to create your own theme for your launcher activity.

Create custom style for activity in  style.xml

    <style name="SplashTheme" parent="Theme.AppCompat.NoActionBar">
        <item name="android:windowBackground">@drawable/splash_background</item>
    </style>

Here I have created splash_background.xml file in drawable, which contains gradient and app logo.

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

    <item android:drawable="@drawable/gradient" >
    </item>

    <item>
        <bitmap
            android:gravity="center"
            android:src="@drawable/ic_logo"/>
    </item>

</layer-list>

The final step, Assign your theme to activity in Manifest.xml file.

<activity
            android:name=".MyActivity"
            android:screenOrientation="portrait"
            android:theme="@style/SplashTheme">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

Now you are ready to rock!