To access Picassaweb directly, you need to go to
https://picasaweb.google.com/lh/myphotos?noredirect=1 (The noredirect=1 keeps it from going to G+ instead). Pick a photo you've uploaded to picassaweb, and on the right you should see a section with "Photo Information", "People", "Photo Location", etc...
One of those will be "Link to this photo"
Click that and it expands and you can see a Link and Embeded Image section, as well as a select size.
Pick the size you want (Small or Medium are usually good for message boards), click the "Image only (no link)" box, and then copy the complete URL in the "Embeded Image" section.
Now in your message board post, create an Image link (the insert image button) and paste that URL in the box that pops up. (or just stick it between (img)(/img) tags (replace ()'s with []'s)