PM > Install-Package Our.Umbraco.Picture

@Html.RequiresJs("~/Scripts/picturefill.min.js", 101);

@{
    IPublishedContent cat = Umbraco.TypedMedia(1000);
    IPublishedContent tiger = Umbraco.TypedMedia(1001);
    var picture = Umbraco.Picture()
        .Source("(min-width:480px)", tiger.GetCropUrl(768, 300))
        .Source("(min-width:320px)", cat.GetCropUrl(480, 300) + " x1", cat.GetCropUrl(960, 600) + " x2")
        .Srcset(cat.GetCropUrl(768, 300))
        .Attr("class", "img-responsive")
        .Alt("Cat becomes tiger");

    @Html.RenderPicture(picture);
}

<picture class="img-responsive">
    <!--[if IE 9]><video style="display: none;"><![endif]-->
    <source media="(min-width:480px)" 
            srcset="/media/1001/tiger.jpg?width=768&height=300&mode=crop">
    <source media="(min-width:320px)" 
            srcset="/media/1000/cat.jpg?width=480&height=300&mode=crop 1x, 
                    /media/1000/cat.jpgwidth=960&height=600&mode=crop 2x">
    <!--[if IE 9]></video><![endif]-->
    <img src="data:image/gif;base64,R0lGODlhAQABAAAAADs=" srcset="" alt="Cat becomes tiger" /> 
</picture>

@{
    IPublishedContent media = Umbraco.TypedMedia(1000);
    var picture = Umbraco.Picture(media)
        .Source("(min-width:992px)", 1200, 300, 1.0, 2.0)
        .Source("(min-width:768px)", 992, 300, 1.0, 2.0)
        .Source("(min-width:480px)", 768, 300, 1.0, 2.0)
        .Source("(min-width:320px)", 480, 300, 1.0, 2.0)
        .Srcset(768, 300)
        .Attr("class", "img-responsive")
        .Alt(media.GetPropertyValue<string>("alt", ""));

    @Html.RenderPicture(picture);
}

<picture class="img-responsive">
    <!--[if IE 9]><video style="display: none;"><![endif]-->
    <source media="(min-width:480px)" 
            srcset="/media/1001/image.jpg?width=1200&height=300&mode=crop x1,
                    /media/1001/image.jpg?width=2400&height=600&mode=crop x2">
    ...
    <!--[if IE 9]></video><![endif]-->
    <img src="data:image/gif;base64,R0lGODlhAQABAAAAADs=" srcset="/media/1001/image.jpg?width=768&height=300&mode=crop" alt="..." /> 
</picture>