Slow speed of second hover image

I am using this custom css plus code block <div id="img-hover"></div> and would like to slow speed so it doesn't jump so quickly from first image to second image. Any help would be much appreciated.

#img-hover{

  background-image:url('https://static1.squarespace.com/static/61cb2383021d34467e693bf6/t/61cdd953e537667a78e5a6bb/1640880473238/Katy+Hepburn_1SSBend1.png');

height:21vw;

width:29vw;

background-size:contain;}

#img-hover:hover{

  background-image:url('https://static1.squarespace.com/static/61cb2383021d34467e693bf6/t/61cdd97310a48011dfc6d8f4/1640880518189/KH+Bend+One+shine+image+1.png')

}

Try add this CSS

#img-hover {
	transition: all 0.9s;
}