I had the problem a week ago, add this to your config.xml:
" <icon src="icon.png" gap:platform="ios" />
<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-76.png" gap:platform="ios" width="76" height="76" />
<icon src="icon-57.png" gap:platform="ios" width="57" height="57" />
<icon src="icon-144.png" gap:platform="ios" width="144" height="144" />
<icon src="icon-114.png" gap:platform="ios" width="114" height="114" />
"
Then add all those images with the correct width and height, that should fix it.