what you do is choose manage attachments. Then choose upload from URL and type in the image address, or if on disk browse for the file on disk. then choose upload. make sure its uploaded and the file isnt to big. close this dialog. then choose the paperclip icon (attach) on the chatbox toolbar and chose the file you uploaded and it should insert automagically into the dialog. choose preview to see if it worked.
you have to use the extended dialog to do all this you cant use quick reply.