Yup, I just found it. Thanks.

Go to "My Active" at the top next to your user name,

Click "Edit Preferences" in the menu on the left

Change the drop-down menu above your time zone from "Flat" to "Threaded"

Click "Save Preferences"

When you are writing a
new post, click the icon that looks like a little Polaroid picture,
between the Chainlink and Letter A icons. It will bring up a window
that allows you to upload a picture; after uploading check the box for
"Insert as Thumbnail" at the bottom and click "Insert" to put the picture
directly into your post. You can also refer to a picture located elsewhere on the internet (such as a Flickr account or VillagePhotos) by clicking the "Web" tab, which will allow you to cut and paste the URL of the photo's location.

In your post, when others click on your thumbnail, the picture
opens in a nice wndow of its own over top of the discussion thread.