I believe that means that the boards skipped the intermediate process of uploading the picture and think that they had the picture to embed, so they're trying to embed a picture that isn't actually uploaded. Try clicking cancel and then re-uploading the image, it might help.
Oh, and if you finish your post but it still says "uploading" even though all of the pictures you uploaded are already embedded (i.e. no [Uploading...]() in the post) then it's trying to embed a non-existent picture. Simply use Ctrl + Enter to post.