What URL to use to have external image using watermark?

Hi,
I'm trying to use the external image block to display a random background image. I want the watermark to be shown also.

I have edited ImageBlock.tpl to return a URL which will display the image (see http://gallery.menalto.com/node/53664), but this does not show the watermark.

The watermark's fine in the album, so it's something to do with the way I'm calling core.DownloadItem, I think. I'm currently using
{g->url arg1="view=core.DownloadItem" arg2="itemId=`$block.id`" arg3="serialNumber=1" forceFullUrl=$ImageBlockData.forceFullUrl}
and I'm guessing there's another argument I can pass to DownloadItem to show the watermark.

Does anyone know how to achieve this? I'm sure it's simple, I just can't track it down!

Thanks for your help,
Mark