spee.ch/views/partials/publishForm-Thumbnail.handlebars

<div class="row row--padded row--wide row--no-top" id="publish-thumbnail" hidden="true">
    <div class="column column--3 column--sml-10">
        <label class="label">Thumbnail:</label>
    </div><div class="column column--6 column--sml-10">
        <div class="input-text--primary">
            <input type="text" id="claim-thumbnail-input" class="input-text input-text--full-width" placeholder="https://spee.ch/xyz/example.jpg" value="" oninput="updateVideoThumb(event)">
        </div>
    </div>

</div>

<script type="text/javascript">
    function urlIsAnImage(url) {
        return(url.match(/\.(jpeg|jpg|gif|png)$/) != null);
    }

    function testImage(url, timeoutT) {
        return new Promise(function (resolve, reject) {
            var timeout = timeoutT || 5000;
            var timer, img = new Image();
            img.onerror = img.onabort = function () {
                clearTimeout(timer);
                reject("error");
            };
            img.onload = function () {
                clearTimeout(timer);
                resolve("success");
            };
            timer = setTimeout(function () {
                // reset .src to invalid URL so it stops previous
                // loading, but doesn't trigger new load
                img.src = "//!!!!/test.jpg";
                reject("timeout");
            }, timeout);
            img.src = url;
        });
    }

    function updateVideoThumb(event){
        var videoPreview = document.getElementById('asset-preview');
        var imageUrl = event.target.value;
        if (urlIsAnImage(imageUrl)){
            testImage(imageUrl, 3000)
                .then(function(result) {
                    if (result === 'success'){
                        videoPreview.src = imageUrl;
                    } else if (result === 'timeout') {
                        console.log('could not resolve the provided thumbnail image url');
                    }
                })
                .catch(function(error) {
                    console.log('encountered an error loading thumbnail image url.')
                })
        }
    }
</script>