If you already have the image somewhere accessible on the web, you can just paste its URL into the box called Optional Image URL: on your new post or reply page. Be sure you don't have the http:// twice at the beginning of the string as the box is pre-filled with that.

If you have the image on your PC, you can use the File Upload facility that you will find in the yellow horizontal stripe the bb gives you when you are posting or replying. There are instructions there on how to do it. Basically, you will want to click the Browse button when you use File Upload, browsing your PC to find the location of the image, and clicking on it. The Upload facility will upload a copy of your image to the Gallery associated with this bb and place a link to that on the message you are composing. That's it, except you should note the limitations on the file types that can be used for File Uploading (.jpg or .gif usually).