var h2 = document.querySelectorAll('h2');

var imgs = {
    'толковый словарь': "https://www.maultalk.com/uploads/av-88965.jpg",
    'словарь Ожегова': "https://www.maultalk.com/uploads/profile/photo-59905.png"
};

for(var i = 0; i < h2.length; i++) {
    var h2_parent = h2[i].parentNode;
    var h2_text = h2[i].innerHTML;
    for (var world in imgs) {
        if (h2_text.indexOf(world) + 1) {
            var image = document.createElement('img');
            image.src = imgs[world];
            h2_parent.insertBefore(image, h2[i]);
        }
    }
}

<script>
var h2 = document.querySelectorAll('h2');

var imgs = {
    'толковый словарь': "https://www.maultalk.com/uploads/av-88965.jpg",
    'словарь Ожегова': "https://www.maultalk.com/uploads/av-59905.png"
};

for(var i = 0; i < h2.length; i++) {
    var h2_parent = h2[i].parentNode;
    var h2_text = h2[i].innerHTML;
    for (var world in imgs) {
        if (h2_text.indexOf(world) + 1) {
            var div = document.createElement('div');
            var image = document.createElement('img');
            image.src = imgs[world];
            div.appendChild(image);
            h2_parent.insertBefore(div, h2[i]);
        }
    }
}
</script>

document.addEventListener('DOMContentLoaded', functions() {
    var h2 = document.querySelectorAll('h2');

    var imgs = {
        'толковый словарь': "https://www.maultalk.com/uploads/av-88965.jpg",
        'словарь Ожегова': "https://www.maultalk.com/uploads/av-59905.png"
    };

    for(var i = 0; i < h2.length; i++) {
        var h2_parent = h2[i].parentNode;
        var h2_text = h2[i].innerHTML;
        for (var world in imgs) {
            if (h2_text.indexOf(world) + 1) {
                var div = document.createElement('div');
                var image = document.createElement('img');
                image.src = imgs[world];
                div.appendChild(image);
                h2_parent.insertBefore(div, h2[i]);
            }
        }
    }
});

var imgs = {
        'толковый словарь': "https://www.maultalk.com/uploads/av-88965.jpg",
        'словарь Ожегова': "https://www.maultalk.com/uploads/av-59905.png",
        'слово': "картинка",
        'слово': "картинка",
        'слово': "картинка",
        'слово': "картинка",
        ...
    };