I think the toolbar button is designed to let you insert a link to an image in your post, but the image already has to be online somewhere. The BB WYSIWYG plugin doesn’t handle uploading or storing images. It just modifies your post.
So, they work in different ways. If you used the BB WYSIWYG editor and the Allow Images plugin together, AND the image was online somewhere already ( like this http://www.google.com/images/nav_logo8.png )then you would be able to use the toolbar button. But if the image is not online yet and you want to store images for your users, then I think bbPress Attachments is the way to go.