Why can't you just use a class for the icon?
Or could you do something like this?
<thead>
<tr data-bind="foreach: tableHeaderNames">
<th>
<a href=";" data-bind="click: $parent.sortReport.bind($data, 'byDate') ">
<img src='@Url.Content("~/assets/images/icons/sort_asc.png")' />
<span data-bind="text: $data.Name"></span>
</a>
</th>
</tr>
</thead>