How to remove "padding" from ionic card

Add margin: 0 !important; and padding: 0 !important; for your card's css. Like this:

ion-card {
width:100%;
height:300px;
margin: 0 !important;
padding: 0 !important;
}

ion-card > img {
 width: 100%;
height: 100%;
}

<ion-card>
<img src="...">
</ion-card>

See 1st you should never assign negative values to padding as it won't work. If you need to work with negative values then use margin,

2nd try adding width and height of 100% to your img.

Just for example -

#parent{
  width:500px;
  height:300px;
}

#parent > img{
  width:100%;
  height:100%;
}

<div id="parent">
<img src="https://source.unsplash.com/random">
</div>