If you’re using IE 10 or newer, press F12. When the debug menu comes up at the bottom, click on “Browser Mode” and select “Internet Explorer 9.” After that, in the same tab, refresh your page. This forces flickr into running the old-style menus with buttons that actually work where you can select your share method and filesize from logical drop-down menus.
Thanks orionid, I use Firefox and got around the problem by moving the picture to photobucket.