Picasa doesn't allow remote hosting I think, but with a normal photo hosting site all you need to do is either copy the image tagged URL from there to here, of if its not provided put the following around the URL. I use
www.postwhore.biz and
www.photobucket.com
[*img]http://www.yoururl.com[/img*] only without the *, I just stuck that in so you'd see the code rather than a red x:)