Ygrec; I think you might be confused by the word "attachment". I think you want to "insert" an image; that can't be done from your computer, it has to be done from a website (like this one), website to website.

J.D. has said she uploads the pictures into her photo album here and then uses that URL/address in the yellow-mountain/postage-stamp looking button "insert image" slot. I don't know if that will work for you. I own a couple of my own sites and upload pictures to that site and use those urls to get the picture. But you can't go from your computer to a post (only your computer to your avatar); you have to upload first to this site's photo album/another website.