Go to your Flickr account, click the photo you want to post from your photostream (initially they show as thumbnail sized images that you can view by clicking). Once the page reloads you should see a "Share This" link above the image. Click that link and select the Grab the HTML/BBCCode. There will be a drop down to select the image size (usually just stick with the default of medium or 500). Select the BBCode radio button and copy that information into your post. No need to wrap the code with the image tags in your post as it does it for you.
IMG_7954 by
yodaearlkarma, on Flickr
Note if you don't want it to include a link to your image or account (note the links right below my image), you can strip the last two lines out and only include the first set of info contained in the first URL brackets
Let me know if that made sense