Go to the photo page of any of your own uploaded photos.

You'll see a button that says All Sizes above it. Click that and you'll see a page that lists all the sizes that photo is available in. To fit comfortably on these pages, choose a Medium (500 pixels wide) or smaller size.

Below the picture you'll see a text box with the code you can select. Copy all of that and paste it into the Comment or Body field here, and press return a couple of times below the code.

Write the rest of your post, submit and voilà!

(This is also how you post flickr photos to any other blogging, forum etc.service by the way.)