The new Windows Phone 8.1 build system has an issue where it prints incorrect warnings regarding icon and splash screen images. This is a known issue that will be fixed. A typical error message is shown below.
The minimum set of icons and splash screens that are required to build a Windows Phone 8.1 package are shown below, see the width and height dimensions at the end of each line:
<!-- Windows Phone 8.1 minimum icons and splash screens --><platform name="wp8"><icon src="pkg/windows/Wide310x150Logo.scale-240.png" width="310" height="150" /><icon src="pkg/windows/Square150x150Logo.scale-240.png" width="360" height="360" /><icon src="pkg/windows/Square71x71Logo.scale-240.png" width="170" height="170" /><icon src="pkg/windows/Square44x44Logo.scale-240.png" width="106" height="106" /><icon src="pkg/windows/StoreLogo.scale-240.png" width="120" height="120" /><splash src="pkg/windows/SplashScreenPhone.scale-240.png" width="1152" height="1920"/></platform>