There are two ways, if you have your picture hosted somewhere on the internet (e.g. "http://www.peanutsmom.com/chinchilla.png"), you can click the "insert image" button on the top of the Reply box and put the url link directly into the box, like this:
If you don't have the images hosted on the internet, and want to upload them as attachments to the post, scroll down to the bottom of the thread, hit "Manage Attachments," then "Choose File", then locate them on your computer:
Hope that helps! Looking forward to seeing lots of adorable chin photos!