It's not really set up to do that, but you could do it pretty easily by subclassing it out. Some pseudo-code:

class MyImageGalleryPage_Controller extends ImageGalleryPage_Controller
{
function image()
{
return $this->customise(array('Item' => DataObject::get_by_id("ImageGalleryItem",$this->urlParams['ID'])))->renderWith(array('MyImageGalleryPage_image','Page');
}
}

MyImageGalleryPage_album.ss:

         <% control GalleryItems %>
            <li style="height:{$Top.ThumbnailSize}px;width:{$Top.ThumbnailSize}px;">
                  <a href="<% control Top %>$Link(image)<% end_control %>$ID"><img src="$ThumbnailURL" alt="$Title"/></a>
            </li>
         <% end_control %>

MyImageGalleryPage_image.ss

<% control Item %>
$Image.SetWidth(500)
$Caption
<% end_control %>

Something like that oughta work.