Make an account, login then, hit reply in the lower right to make a comment  
To insert a picture, code  or a link use the buttons where you type a message.
For a picture, upload the picture to Imgur.com or a similar service. Then, copy and paste the
direct link to the picture. Highlight the link with your cursor and click the insert image button
If unsure, you can always click "quote" on a message similar to what you want to do and see how they did it.