Yup, I've been able to make it work after a lot of retries/errors. In the root folder, I added the following icons (with right sizes)
icon-16.png
icon-32.png
icon-57.png
icon-76.png
icon-114.png
icon-120.png
icon-128.png
icon-152.png
icon-256.png
icon.png (size 57x57 px)
and in the XML file, I pasted the following code :
<icon src="icon.png" gap:platform="ios" width="57" height="57" />
<icon src="icon-128.png" width="128" height="128" />
<icon src="icon-256.png" width="256" height="256" gap:role="default" />
<icon src="icon-57.png" gap:platform="ios" width="57" height="57" />
<icon src="icon-120.png" gap:platform="ios" width="120" height="120" />
<icon src="icon-152.png" gap:platform="ios" width="152" height="152" />
<icon src="icon-114.png" gap:platform="ios" width="114" height="114" />
<icon src="icon-76.png" gap:platform="ios" width="76" height="76" />
I guess everything is not mandatory, but at least, it works for me!
I hope it will work for you too!