Sorry for the delayed response.
The ability to post attachments is limited to those users who have been approved for "full member" status. Stick around and participate for a bit, and it generally doesn't take long for that to happen.
Once users participate enough to become "Tilted" rather than "Upright," they are flagged for review by the moderators, who then handle promoting the users to "full member." This is in no way meant as a punishment for new users, but rather an encouragement to actively participate in the forums for a little bit before contributing to the site's bandwidth costs