tweetdeck_paste_image.js

document.getElementsByClassName('compose-text-container')[0].onpaste = function (event) {
    var items = (event.clipboardData  || event.originalEvent.clipboardData).items;

    // find pasted image among pasted items
    var blob;
    for (var i = 0; i < items.length; i++) {
        if (items[i].type.indexOf("image") === 0) {
            blob = items[i].getAsFile();
        }
    }

    // load image if there is a pasted image
    if (blob !== null) {
        var reader = new FileReader();
        reader.onload = function(event) {
            media_el = '<div class="compose-media-bar-holder padding-al">' +
                            '<div class="compose-media-bar">' +
                                '<a class="js-media-bar-remove compose-media-bar-remove is-actionable">' +
                                    '<i class="sprite sprite-close"></i>' +
                                '</a>' +
                                '<img class="js-media-bar-thumb compose-media-bar-thumb full-width" alt="" src="' + event.target.result + '" style="height: auto;">' +
                            '</div>' +
                        '</div>';
            
            var media_element = document.getElementsByClassName('js-media-added')[0];
            media_element.className = 'js-media-added';
            media_element.insertAdjacentHTML('beforeend', media_el);
        };
        reader.readAsDataURL(blob);
    }
}