I am not sure I understand the question and what exactly you are trying to accomplish here, but the startup image (splash screen) is displayed by the os while the app is being loaded into memory. Therefore the duration is controlled by the os.
If you want the splash screen to display longer, then you would need to:
1. Setup a splash screen in mobi and set it as the startup screen
2. Initiate a timer on startup for the duration you want it displayed
3. Call gotoscreen when the timer ends to display the next screen