How do I get the web app logo to appear one way and the same across all devices?

I have a web app for mobile and currently the icon that is used in the 'save to desktop' is a png on a black background.

The problem I have is that when I save to home screen on an Android I get a white or clear background and on an iphone I get a black background.

How do I get the saved icon images the same across all devices - a white background, but the 'save to home screen' image to be a png - clear background?

The code currently being used is: