Glide.js, how to check size of image?

I am using Glide.js to create a slider and I have a problem, I don't know how to check size of image inside slide. I must display picture in this slide in actual ratio, i mean if picture have height larger then width I should use in css height:100%; width:auto; In other way if width is larger than height it should be width:100%, height:auto. And this is my problem, I don't know how to do this. I am a begginer in javascript. I can write something in jQuery, but i don't know if this is correct. My slider look like:

<div class="slider" id="paintSlider">
                                    <ul class="slides">
                                        <li class="slide paint">
                                            <span> <img src="img/glowna_tlo.jpg" alt=""> </span>
                                        </li>
                                        <li class="slide paint">
                                            <span> <img src="img/NORWAY-FIRE-COMMUNICATION.jpg" alt=""> </span>
                                        </li>
                                        <li class="slide paint">
                                            <span> <img src="img/taipei-fireworks.png" alt=""> </span>
                                        </li>
                                        <li class="slide paint">
                                            <span> <img src="img/00003761.jpg" alt=""> </span>
                                        </li>
                                        <li class="slide paint">
                                            <span> <img src="img/super-squirrel.png" alt=""> </span>
                                        </li>
                                        <li class="slide paint">
                                            <span> <img src="img/searing-mountain.png" alt=""> </span>
                                        </li>
                                    </ul>
                                </div>

JQuery:

`var imgS = $("#paintSlider slides slides img");

    var wysokosc = imgS.height();

    var szerokosc = imgS.width();


    if(wysokosc > szerokosc){
        imgS.css(["height","100%"] ["width","auto"]);
    }
    else if(wysokosc<szerokosc){
        imgS.css(["height","auto"] ["width","100%"]);
    }
    else{
        imgS.css(["height","100%"] ["width","100%"]);
    }`

Could you tell me how to do this :