I'm trying to use cordova directly.
The following configuration can be used to define a single default icon which will be used for all platforms.
<icon src="res/icon.png" />
But I've tried this and failed already.
Generate appropriately sized icons as necessary for the platforms you support. I advise against using a single tag to define all your icons and splashes.
You mention PhoneGap Build in your OP, but Cordova in your second. Are you using the CLI or PGB?
I generated the correctly sized icons.
I want to use the default one-size-fits-all icon for testing. I've been told that should work, unless there's a specific reason it shouldn't?
I'm using PhoneGap Build, but because the icon issue isn't clear, I'm trying Cordova and using it for reference - especially as PhoneGap Build references Cordova and generates the Cordova icon during the build.
As regards the splash screen, my reading brought me here: Splashscreen - Apache Cordova