I can never work out the code
This thread, a year ago. Not totally helpful, actually.
I usually just use the IMG button above the post box. Enter the URL and it automagically surrounds the URL of the image with the IMG and /IMG tags (each tag surrounded by square brackets).
If the image is on Flickr, you can either grab the URL of the image directly and use the IMG tag as above, or if sharing is enabled, select the image size of about 500 pixels wide and use Flickr's BBCode option. That chunk of code can be pasted directly into here.