i don't think you can, unless you are a supporting member or a 1st 1000 member (like me). am i right? or maybe you have a limited #. i'm not sure.
but here's how you do it: when you reply to a thread, go down to the additional options and select manage attachments. from there you are free to upload the pic from the desired source. close that window, hit submit reply and voila! there's your thumbnail pic.