You can also place a link to wherever the photo is hosted (if it's online). This will show the thumbnail in the chat window, as well as link them directly to the original host site. Take your pic(k) as to which method you would like to use
Well I guess by now, you have already got your answer several times. However I would suggest you to just post the site link while you are chatting and your website's thumbnail image and description will automatically show up in the chat. Also if the picture size is big, then better to send it in a message