I'm not sure you can embed photos from Picasa on forums. However there
is an option for posting a link to a photo.
There should be a "Link to This Photo widget" on the side of the screen for creating a link that people can click through to see your photo.
Here is a help faq about sharing photos:
https://picasa.google.com/support/bin/answer.py?hl=en&answer=66969