How to filter what is showed based on radio button checked
You will want to first create an 'elementCriteriaModel' to retrieve only the 'featured' images from the 'thumbnails' asset group, which will return an array like object (not technically an array) which you can loop through to display.

{% set images = craft.assets.group('thumbnails').featured(1) %}
{% for image in images %}
    &lt;div class="rollover-item wd ux dv print"&gt;              
        &lt;img src="assets/img/{{ image.filename }}" alt="{{ image.filename }}"&gt;
        &lt;a class="preview" href="work/{{ image.clientname }}"&gt;{{ image.caption }}&lt;/a&gt;
    &lt;/div&gt;
{% endfor %}


Another method that might be easier to manage from a client perspective is to add an assets fieldType to your homepage section (called 'featuredImages' or whatever; limited of course to the 'thumbnails' asset group) which the client can use to add and/or reorder the featured thumbnails to be displayed. In which case the code would look something more like this.

{% if entry.featuredImages|length %}
    {% for image in entry.featuredImages %}
        &lt;div class="rollover-item wd ux dv print"&gt;              
            &lt;img src="assets/img/{{ image.filename }}" alt="{{ image.filename }}"&gt;
            &lt;a class="preview" href="work/{{ image.clientname }}"&gt;{{ image.caption }}&lt;/a&gt;
        &lt;/div&gt;
    {% endfor %}
{% endif %}