I use PBASE for all my forum posts.
You have to use the url from viewing the image in PBase inside the tags (replace the "*" with [ and]) it looks like this:
*img*http://www.pbase.com/jimpoor/image/99715516/original.jpg*[/img]
When you replace the "*" you get this:
You also need to include the /original or /medium, etc for PBASE links.
That said, I believe you have direct linking blocked on your PBASE account, so it won't work.