Hi Hans

There is an open issue for adding default class names to the div's in the grid layout:

http://issues.umbraco.org/issue/U4-6585

and a potential fix here: https://github.com/umbraco/Umbraco-CMS/pull/678

that I've been using recently on sites with a grid layout.

I'm not sure about setting a default class on an img, other than updating the framework rendering file in /views/partials/grids/boostrapetc.cshtml

or potentially customize the /views/partials/grid/editors/media.cshtml file that writes out the html for a media item:

@model dynamic
@using Umbraco.Web.Templates

@if (Model.value != null)
{   
    var url = Model.value.image;
    if(Model.editor.config != null && Model.editor.config.size != null){
        url += "?width=" + Model.editor.config.size.width;
        url += "&height=" + Model.editor.config.size.height;

        if(Model.value.focalPoint != null){
            url += "&center=" + Model.value.focalPoint.top +"," + Model.value.focalPoint.left;
            url += "&mode=crop";
        }
    }

    <img src="@url" alt="@Model.value.caption" class="my-default-super-responsive-grid-css-class">

    if (Model.value.caption != null)
    {
        <p class="caption">@Model.value.caption</p>
    }
}

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