Simply click on the button on the far right above the "send" button. Two options will pop up, the first being the option to upload a photo from your computer, and the second option which is to post a photo from the internet by url.

To upload a photo from your computer just click on the "upload image" option and select your photo.

To post a photo from the internet just type in the url to the photo such as "http://www.somesite.com/photo.jpg

Remember that only JPEG images can be posted to the chat room.