I see.
The difference between the two is that the ones that don't work don't have the 'www' in the url. The short answer is that you should access the site with the www. in the url and if you've copied a link to an image, put it in there before putting it in your reply.
The technical reason has to do with a change made to allow people to access the site using HTTPS - at
PlanetCricket Forums - part of this required that all embedded items, such as images in a post, are loaded off a secure server. So now images in posts are loaded via the PlanetCricket server. In this case, because the url didn't match, it was trying to load an image on the server, via the thing for loading images from other sites, and that broke.