I think you need X number of posts to your credit before it works for you.
I may be wrong but I seem to recall members with 'Newbie' status can't post attachments directly yet.
More of an Off Topic/ non-RB post though, just sayin; it might get more attention and explanation in the other Topics.

/there are also file size considerations. Too big a file won't post as an image .. I've ran into that before also. Not just file size as in under 2 meg, but also actual image size being over say 1200px.
Make your sound your own!