Taken from the Twitter's API section.
Why do my image uploads always fail? The image update methods require multipart form data. They do not accept a URL to an image not do they accept the raw image bytes. They instead require the data to be delivered in the form of a file upload.
Has anyone came to a conclusion with this, or resolved this issue? I'm having various amounts of trouble trying to get it to post an image. I've looked around and found no solutions with this.