David, I will need Cpanel access details and admin access details fo the website, as you can see there's no such issue on our demo, which is also 1.7.2 and activity notifications is 8531 as well.
Can you check the server error when you try to directly open the image? Maybe with tools like Firebug/Yslow for Firefox in "components" tab. Could be a cache or restriction error, too.