Does anyone know why I can't seem to render images using fo:external-graphic?  I've got the following code which loops through any images stored inside a folder:

@{
   var mediaFolderId = Model.itemImageGallery;
    var folder = Library.MediaById(mediaFolderId);
}

and then:

<fo:block font-size="10pt" margin-bottom="15mm">
   @{
     if (folder != null && folder.Children.Count() > 0)
      {    
           foreach (var media in folder.Children)
         {
              <fo:block>
                   <fo:external-graphic src="@media.umbracoFile"></fo:external-graphic>
               </fo:block>
          }
      }
  }
</fo:block>

If I remove the <fo:external-graphic> tag and just have @media.umbracoFile in the block then it renders the path to the image fine, i.e.

/media/1012/file01.jpg
/media/1012/file02.jpg
/media/1012/file03.jpg
/media/1012/file04.jpg
etc...

So I'm not sure why it then doesn't render anything when writing: 

<fo:external-graphic src="@media.umbracoFile"></fo:external-graphic>

I'm clearly missing something here. Any ideas?

Many thanks,
Mike