crop an image in a slider animation with html/css

I have been trying to make an image slider that would fit into a side bar in my website im building, however, its been coming out with different pictures with different aspect ratios making it look all messy. I want to try having it where it will automatically crop (maybe even round it) with my current slider I'm using. Thanks in advance!

<head>
<style type="text/css">

#slider {
    overflow: hidden;
}
#slider figure {
    position: relative;
    width: 500%;
    margin: 0;
    left: 0;
    animation: 20s slider infinite;
}
#slider figure img {
    float: left;
    width: 20%;
}

@keyframes slider {
    0% {
        left: 0;
    }
    20% {
        left: 0;
    }
    25% {
        left: -100%;
    }
    45% {
        left: -100%;
    }
    50% {
        left: -200%;
    }
    70% {
        left: -200%;
    }
    75% {
        left: -300%;
    }
    95% {
        left: -300%;
    }
    100% {
        left: -400%;
    }
}

</style>
</head>
<body>

    <div id="slider">
        <figure>
            <img src="img/plant.png">
            <img src="img/plant2.png">
            <img src="img/plant3.png">
            <img src="img/plant4.png">
            <img src="img/plant.png">
        </figure>
    </div>


</body>
</html>

Solution

The trick is to force all images to inherit the height of the parent element by min-height: 100%; and height: auto; and then use object-fit: cover; to keep the aspect ratio.

#slider {
    overflow: hidden;
    height: 180px; /* or any value depending on how high you want it to be. 100% for matching the longest image */
}
#slider figure {
    display: flex;
    width: 500%;
    height: 100%;
    margin: 0;
    position: relative;
    animation: 20s slider infinite;
}
#slider figure img {
    display: block;
    width: 20%;
    min-height: 100%;
    object-fit: cover;
    height: auto;
}

@keyframes slider {
    0% {
        left: 0;
    }
    20% {
        left: 0;
    }
    25% {
        left: -100%;
    }
    45% {
        left: -100%;
    }
    50% {
        left: -200%;
    }
    70% {
        left: -200%;
    }
    75% {
        left: -300%;
    }
    95% {
        left: -300%;
    }
    100% {
        left: -400%;
    }
}

    <div id="slider">
        <figure>
            <img src="http://placekitten.com/700/300">
            <img src="http://placekitten.com/600/600">
            <img src="http://placekitten.com/500/300">
            <img src="http://placekitten.com/700/400">
            <img src="http://placekitten.com/700/300">
        </figure>
    </div>

and one more thing, position: relative; does not take left, right, ... values. That works only on absolute and fix positioned elements.