When you click to view an image in Flickr there will be a "Share" icon on the right hand side. It looks like a square with an arrow pointing out of it. After clicking on it a pop-out box will appear. Click the "BB Code" link then select the image size. I prefer "Medium 640x480" or "Medium 500x374". Copy the text below the link into your post and the image will magically appear when you click "Preview" or "Submit".
The code for posting will look something like this: