You`ll first need to scan the sketch/drawing using a flatbed scanner or take a good digicam photo of it,saving the file as either a JPEG or GIF file format of max. width 480 pixels.
You`ll need a photo editing program to do this.
To attach to a forum post simply compose a post as normal but check the Preview/Add attachment box and then browse your hard drive for the correct file and submit your post.
The attachment should show as blue legend in your post although i believe Steve is playing about with a auto attach function to insert pictures directly into posts.