If you want to use a picture from a website, first right-click on the picture. Then, left click on properties. Highlight and copy the location line (if using Firefox; if using IE, copy the 'Adress (URL) line).
Now, when you want to insert the image in a post, click 'Img', then paste your image location, then click 'Img' again. That should do the trick.
If you want to insert an image from your own hard drive, it's slightly more involved. Go to an image hosting site (I use
http://tinypic.com/). Use the browse button on their page to find and select your picture. Then ,click on 'Upload now'. Finally, when your picture has uploaded, copy the details from the line marked "IMG Code for Forums & Message boards". Then, when creating your post, simply paste that line (no need to click on 'Img' at the beginning or the end; that has already been taken care of at the Tinypic stage(). Good luck.