I think that icon means the image cannot be displayed. There are a lot of reasons why that might be the case.
Maybe the image is a file type that isn't compatible with your web browser, operating system, etc.
Maybe the posters bungled the URL.
Maybe they posted an image that can only be viewed if you are logged in to the the site that hosts the image.
Maybe the image hosting site does not allow directly linked images.
Maybe the original image has been removed from the site for a variety of different reasons (including violation of terms of use, copyright issues, illegal content, etc.).
Maybe your network provider (especially if you are surfing from work) has blocked access to specific photo sharing sites.
Those are just some of the reasons why a posted image might not display here.