In order to attach a picture, you first need to upload it to a website and use < image url > to attach it to your post.
To fix this problem, you can try to clear the cache and history from your browser. This should work most of the time. Also, try to update your add-on to the latest versions. If none of these are working, try using another browser like Internet Explorer for a while. The cause for this is currently unknown but some people fix it using the mentioned solution.