Do you happen to know which link to post?
Confession time: when I started posting pics on the forum, it took me ages to work out how. I just kept copying and pasting the different links into the post until one of them worked.
The reason I like Postimage (apart from it being free) is that it offers you half a dozen different links, each clearly labelled ‘for forums’ ‘for thumbnail image’ or ‘for email’ and so on.
I have used photobucket and a couple of others too, so they all offer a similar service, although some charge.