It's no longer as easy as it was until a week or two ago.
You choose the photo, click on the little magnifying glass which says (when you hover the cursor over it) "move to showcase" (or whatever it says in English, my Flickr speaks German with me), the photo opens up against a black background and you find "show all sizes" in the upper righthand corner. You click on that, open at the size you want to, right-click the photo, copy the URL (which MUST end in -.jpg!), open your post, (manually) put down
From your photostream click on a photo. Click "Share this."
Click "Grab the HTML" then below the box with the URL in it, click "BBCode" then from the pull down menu choose your size (I generally use large here) and copy/paste the resulting code into the reply box here. Pretty easy.