No, you can't do this from iPhoto (unless you publish it to a Mobile Me website through your dotMac account). In iPhoto, all you need to do is export the photo(s) you want to use to your desktop. Then go to Photobucket.com (or similar site) and use their uploading tools to upload the photo(s) from your desktop to their website. Then you will be able to get a URL that you can copy and paste into your forum post. Hope that helps!