This will rerender the way Markdown handles image tags by using Marked's ability to customise the rendering. This particular function enables users to enter dimensional values (such as width and height) as well as class and id selectors to the image tag.

render-image-markdown.js

renderer.image = function( href, title, text ) {
    var result = ' alt="' + text.trim() + '" '; // this will be used to store our dimension string (if applicable)
    var w, h; // our width & height variables

    // title tags need to be between double quotes
    var rTitle = /"(\w*\s*)+"/;

    // dimension values need to prefix with an equal sign
    // followed by at least one digit and the times 'x' sign
    // and (optionally) MAY have a second value
    var rDims = /=(\d+)x(\d*)/;

    // classes to be placed within the image need to be prefixed with a colon
    // any subsequent classes to be added with a "." without any leading or trailing
    // spaces. IT MUST HAVE A SPACE IN FRONT OTHERWISE IT WILL CONFLICT WITH
    // LINKS CONTAINING http://...
    var rClass = /\s+:([^\)|\s]+)/;

    // ids need to be prefixed with a hash symbol. MUST HAVE A SPACE BEFORE OTHERWISE
    // CAN GET CONFUSED WITH URLs.
    var rId = /\s+#([^\)|\s]+)/;

    // first let's grab the title and then remove from href
    if ( title === null ) {
        var getTitle = rTitle.exec( href );
        if ( getTitle && getTitle[0].length ) {
            title = getTitle[0].replace( /"/g , "" );
            result += ' title="' + title + '" ';
        }
        href = href.replace( rTitle, "" );
    }

    // get dimension values from href
    var getDims = rDims.exec( href );
    if ( getDims[0].length ) {
        // width is the first dimension value
        w = getDims[1];
        result += ' width="' + w + '" ';
        // height is the second dimension value (optional)
        if ( getDims.length > 1 ) {
            h = getDims[2];
            result += ' height="' + h + '" ';
        }
        href = href.replace( rDims, "" );
    }

    // get class tag from href
    var getClass = rClass.exec( href );
    if ( getClass && getClass[0].length ) {
        var c = getClass[1];
        result += ' class="' + c + '" ';
        href = href.replace( rClass, "" );
    }

    // get id tag from href
    var getId = rId.exec( href );
    if ( getId && getId[0].length ) {
        var i = getId[1];
        result += ' id="' + i + '" ';
        href = href.replace( rId, "" );
    }

    return '<img src="' + href.trim() + '" ' + result + '>';

};