This is because the steemit.com website use an image proxy that caches the images from the internet.

If the URL of the image does not change, then the proxy deliver the image it stored rather than downloading it from the original source. Unfortunately, the proxy does not check if the referenced image has been updated.

As SteemitBoard's images URL does not change, steemit.com always display the first image it has downloaded.

I am in contact with the Steemit devs to find a solution.