Icons and splashes both are specified to some platform like android, iOS, windows, blackberry. We can define a icon or splash for a particular platform by the following ways:

By specifying the platform attribute:
 

<icon src="icon-60@3x.png" platform="ios" width="180" height="180" />

 By giving icon or splash inside a tag named platform:
 

<platform name="ios">
    <icon src="icon-60@3x.png" width="180" height="180" />
</platform>

Icons:

You can find icon tag inside your config.xml file. It must have the following attributes:
src: It identifies the path of the image file.
width and height

For iOS:

<!-- iPhone 6 / 6+ -->
<icon src="icon-60@3x.png" platform="ios" width="180" height="180" />

<!-- iPhone / iPod Touch  -->
<icon src="icon-60.png" platform="ios" width="60" height="60" />
<icon src="icon-60@2x.png" platform="ios" width="120" height="120" />

<!-- iPad -->
<icon src="icon-76.png" platform="ios" width="76" height="76" />
<icon src="icon-76@2x.png" platform="ios" width="152" height="152" />

<!-- Settings Icon -->
<icon src="icon-small.png" platform="ios" width="29" height="29" />
<icon src="icon-small@2x.png" platform="ios" width="58" height="58" />

<!-- Spotlight Icon -->
<icon src="icon-40.png" platform="ios" width="40" height="40" />
<icon src="icon-40@2x.png" platform="ios" width="80" height="80" />

For Android:

<icon src="ldpi.png" platform="android" qualifier="ldpi" />
<icon src="mdpi.png" platform="android" qualifier="mdpi" />
<icon src="hdpi.png" platform="android" qualifier="hdpi" />
<icon src="xhdpi.png" platform="android" qualifier="xhdpi" />
<icon src="xxhdpi.png" platform="android" qualifier="xxhdpi" />
<icon src="fr-xxhdpi.png" platform="android" qualifier="fr-xxhdpi" />

Splash screens:
This is also inside your config.xml file. It can have attributes like src, height, width.

<splash src="splash/ios/Default-568h@2x~iphone.png" platform="ios" width="320" height="480" />