You do not.
The Construct 3 splash is not customisable.
The main question here is actually why is your "loading-logo.png" failing to show, as it is the way to actually have a customised picture showing.
What I can tell, from testing Glokar in remote preview on my phone is that the logo.png took a bit of time to download/display, but did eventually, right before skipping to the main layout.
The logo is supposed to be downloaded first and be displayed quite quickly.
If it does not on an exported project, perhaps it could be worth it making a bug report for it to the GitHub issue tracker here:
https://github.com/Scirra/Construct-3-bugs/issues