On forums, there are at least a couple more reasons the picture might not display.
If people have made an error, however minor, when copying the 'url' of the picture into their post, the picture won't display.
If the picture they've posted is on a 3rd party website which doesn't allow direct linking.
eg Sometimes the owners of websites put brackets somewhere in the 'url' of their pictures. The brackets don't stop the pics displaying on their own site, but they stop direct linking from other sites.
(There is a way of getting around this but there's more to it than just removing the brackets.)