When running in Windows you don't use the 'assets' directory. The 'assets' directory is only for Android.
I am now assuming that you are using the web sort of one-click to create your apk file. If that is correct, then the zip file you create to upload has the directory 'media' in it (not just the contents, but the actual directory). In that case, you would put the AGKSplash.png file in the zip file at the same level as the 'media' directory. So, when you look at the zip file, you see 'AGKSplash.png' and 'media' at the top level.
At least I assume the online apk builder is set to handle the AGKSplash.png file. I could be wrong about that.
Cheers,
Ancient Lady
AGK Community Tester and AppGameKit Master