There are several reasons why red x's (which generally depict a broken link) can show up in posts.

1. Using a URL to an external image that no longer exists.
2. Copying/pasting a previously used URL and editing a character or using the wrong [] tags.
3. Using a URL to a file uploaded through the old uploaded in the top menu when the file is more than 2 weeks old.
4. Using a URL to a file uploaded through the inbuilt browser in a prior post, and then deleting that prior post. (images uploaded through the inbuilt browser will stay permanently - or until I run out of room - unless you delete the post in which they were first uploaded, in which case the images are also deleted.) So you make a post, see something wrong, re-post with the old URL's, preview and it is fine. Hit the post button and then delete the original post. The images disappear
It is actually quite logical.