It's not possible to replace the splash screen image in the emulator or in the App Preview tool. You can only replace the splash screen in an APK build that you load onto your device or submit to the Google Play store.
In the current version of the XDK (November, 2013) you provide a custom splash screen image at build time. When you go through the build process you turn your HTML5 app into an APK that can be installed on an Android device or submitted to the store. During this build process there is an "Assets" screen that includes options to submit custom splash screens and a custom icon.
If you submit these custom images during the build you will still see the Intel XDK splash screen when you debug your app with App Preview or the built-in emulator. However, when you install the APK that you receive after doing a build it will contain your custom splash screen.