Gallery Widget Template SingleItemLink Thumbnails

Hi, I am wondering if there is a way to specify the thumbnail image of list items in the lightbox gallery template. I want to use the full sized image instead of stretching the thumbnails that are auto generated by Sitefinity.

I tried to set the imageurl property but it still gives me the thumbnails while the href is correct
What Im using:
<sf:SitefinityHyperlink runat="server" id="singleItemLink" ImageUrl='<%# Eval("MediaUrl")%>' rel="prettyPhotos[photos]" class="thumb img" style="display:block; position:relative;" href='<%# Eval("MediaUrl")%>' />