Is your picture on your computer or hosted somewhere on the internet? I think your post means it's on your computer, but just in case, directions for both...
If it's on your computer (and is 800 x 600 or less in size), use the "manage attachments" button below the thread window when you're creating your post to open a window to upload your picture. Browse to find your picture, select it, and click "upload". If the picture is too large it'll reject it, otherwise it'll then show the photo as being uploaded. Click "close this window" and finish typing your post. If you preview the post, the photo won't show as a rule, but if you submit it, it will be there.
If the picture is on the internet, you need its address (URL). To insert it into the thread type the following (removing the spaces):
[img ]http://yourserver.com/yourpicture.jpg[ /img]
inserting the URL within the IMG tags.