It might be due to drag-and-drop, since both Imgur and Vanilla use drag-in-drop for image uploading. That would also explain why one link goes to Imgur, and another to the forums server.
Edit: Unfortunately, its part of the Advance editor and there is no great way to disable it, that I can see anyway. I might be able to modify the Advance editor though, but I haven't looked into it yet. I suppose we could also disable drag-and-drop for the Imgur plugin to resolve the issue of double image uploads.