Here is a good explanation. Thanks to Superduty
To post an image, do the following:

a) get your image in your gallery.
b) open the image you want to use in a post in its medium size. The page you view should look something like this, with your image, of course.



c) find the UBB code on the image page. It is highlighted in blue in the example that I've shown above.

d) highlight the image UBB code and use the "Copy" function to copy it to your clipboard.

e) close the images as required. Go to the forums section and compose your post. Use the "Paste" function to paste the UBB code into your post where you want the image to appear.

f) preview your post. The image should appear. Submit your post. It should view with the image in it.