Updating the BIOS Splash Screen image on the Falls City (FC) and Sterling City (SC) versions of the Intel Compute Stick (ICS) is not supported. It *is* possible to do so, however, on the higher-end (Core m3/m5) Cedar City (CC) ICS. This is accomplished using the new Intel Integrator Toolkit v6 application.
As for locking down Windows so that it can only run one specific application, read this (Microsoft) article: Lock down Windows 10 to specific apps (Windows 10).
Hope this helps,