You need to click to "All Sizes" of your desired picture in your Flickr photostream.
Right mouse click on the larger image and select "copy image location or link" (something similar). This will copy the URL of the image into the clipboard (or memory).
Click on this icon:
on the toolbar at the top of the message box.
And paste the URL into the dialog box. (right mouse click - paste)
Thank you Jasonics. In this case, is the pic that I uploaded into Flickr still too big or is there anyway I can post a smaller pic? Do I need to resize the pic and re-upload to flickr? I think this is too big though. Thanks in advance.