We have recently moved the site to SSL encryption of traffic between the user and our servers. To do this, we have had to ensure that absolutely everything served from the site is also encrypted, including things like ads and third party embedded images. Since we have no control over how the the third party images are served, we have to take a copy of the image, and then serve it directly from our own servers via SSL. To reduce the load on our servers, we store those images for a period of five days in a cache. This means that for five days, you will see the cached version of the image, even if the original source file at the third party has been modified. This appears to the issue you are seeing, with the NHC essentially using the same image, but updating it.
I will look at some other options to see how we can improve on this.