For the GameGuru splash screen you need to go to: C:\Program Files (x86)\Steam\steamapps\common\Game Guru\Files\languagebank\english\artwork\watermark and changed those to what you want.
If you are looking to change the menu pictures, they would be in C:\Program Files (x86)\Steam\steamapps\common\Game Guru\Files\titlesbank\default
You need to change these before you build your standalone game. If you already built the standalone, delete the folder and remake the standalone with the new artwork. Works for me