Short Answer:
You can upload an image or video to a channel via the image and video pickers in the text box at the bottom.
Steps:
1. Click Discussions in the main menu.
2. Click on the channel you want to upload an image or video to.
3. Click the Insert Image or Insert Video button above the text input area.
4. Choose the image or video and click Insert.
5. Note that images and videos will process briefly before being visible to others.