Does iPhoto give a image link?
Sorry, I don't know! If you switch to photobucket, I know how to use that!
You have to click the little picture button that has a square around it and then paste your link in there. At least if you are trying to put one in your post or siggy you have to do it that way. OR you can put "[img]" "[/img]" on either side of it and it automatically tries to make it a photo.
For your profile pic, it might be too big. I always have to shrink my photo down (have no idea how you would do it on iphoto but there's probably a way) so that it's an icon size and then upload it.