Go into your photostream
Click on the image you want to post up
click on "all sizes " above the image
Then on the page that loads, at the bottom is a box that says grab the image url, click in the box and it'll automatically copy.
Now create your post.
above the box you are typing there`s a liitle yellow box ,( insert image ) click that then paste the url you've just copied from flickr
bear in mind that the max image size here is 800x600 so you might want to use the url from the "medium" size image
Hope that explains it well enough