If you have a blog, just copy the URL of the picture from the blog -- then click the IMG box in the forum text editor -- it pops up whenever you type a message -- like this:
This was our boondocking site in the Sequoias, on Halloween.
A couple of tips -- when you click "img" and then paste in your URL, make sure that the pasting does not cut off the "close bracket" after "img." And then after the URL, click "img" again. It should insert a forward slash followed by the letters "img." Whenever you click "img," an example of the proper code pops up on the top of the dialog box.
Not the most user-friendly picture posting system, but it works, most of the time!