If you have saved to your computer, use the third party free site, such as tinypic.com -- there are others for personal preference. From tinypic.com, upload the pic from your computer with the "browse" for file function and set the "resize" right below the "browse" for file due to "default" may not be what you want. After you hit the "upload now" icon, there will be a "IMG code for forums and message boards" -- copy and paste that code into your message.
That is one way that works.