Hit the "Post Reply" button.
Scroll down to "Additional Options", see the imageshack option? hit the browse button to scan the directory where you have the picture saved, when you find it, hit ok/enter, then hit the "Host it" button. This might take a moment or 2, when it finishes you picture will be displayed on a Imageshack web page. There you hace a few options of how you can get it on the webpage here.
Do you know how to upload a picture in a thread?