<img id="map" src="img/bangabandhu111-1170x660.jpg" alt="">

<button type="button" onclick="zoomin()" class="icon"><i class="fas fa-plus"></i></button>
<button type="button" onclick="zoomout()" class="icon"><i class="fas fa-minus"></i></button>

<script>
    function zoomin() {
        var myImg = document.getElementById("map");
        var currWidth = myImg.clientWidth;
        if (currWidth == 2500) return false;
        else {
            myImg.style.width = (currWidth + 100) + "px";
        }
    }
    function zoomout() {
        var myImg = document.getElementById("map");
        var currWidth = myImg.clientWidth;
        if (currWidth == 100) return false;
        else {
            myImg.style.width = (currWidth - 100) + "px";
        }
    }
</script>