To post an image in your chat group page, put the following code in the large text area near the bottom of your group edit page:
<img src="http://somesite.com/someimage.jpg">
(Take out the red text and put the direct link to an image)
To post a video, go to youtube.com and find one you want and get the embed code, copy it, edit your group and paste the embed code in the large text area near the bottom of your group edit page.