No, it's normally a referrer issue. When you request an image file from a web server, it's normal (but not mandatory) to include a reference to the page which "referred" to the image ie the referrer, in the request. Some web servers are configured so that they won't return an image if the referrer isn't one of their own pages. Often, they will return the image if no referrer is supplied, since some web browsers, and some ISPs don't supply a referrer.
So, if you cut and past the URL into a window, the image is returned, since there is no page to be a referrer. Once the image is in your local cache, you will often then see it in the thread (until you clear your cache). The original poster often sees it fine in their view of the post for the same reason, which is why they don't realise there is a problem.
It's an issue with certain web sites, which don't like you using their images on your page, without the rest of their site (often including the advertising which is were they get their revenue from).
