Why can't I attach an image or a PDF file when I reply to an existing topic at the critique section, but when I start a new topic here I can?
Ummm...you cant. Unless once typophile gets out of Beta, they will re-enable this, as it existed on the only typophile.
A lot of folks have a Flickr account, and link.
The only way, at present, to attach files in a thread not created by yourself is to host the pdf and/or file at an alternate server and post the link for download here. It is in the queue of most requested items to be returned.
I see. Thanks.