Resizing Images in Kindle

I'm using Sigil on a Mac. The following CSS works perfect in Apple Books

.x--w50 { width: 50%; }
.x--w40 { width: 40%; }

.div--img { margin: 10px auto; font-family: sans-serif; font-size: 85%; text-transform: uppercase; text-align: center; }
.div--img img { display: block; width: 100% !Important; }
.div--img img { padding-bottom: 5px; border: none; }

<div class="div--img x--w50 alignright"><img src="../Images/bison.svg" alt="Bison"/></div>

However, when I use Calibre to convert my epub to AZW3, it fails. All images display as normal size when previewed in Kindle on my Mac. Does Kindle require some special CSS to resize images?

Thanks.