I'll start this one off:
1. Create a new post -OR- Reply to an existing post
2. Click on Additional options...(at the bottom of the screen)
3. Browse on your computer to the picture you want to add (recommended dimensions 640 by 480)
4. Type in a description into the Message Window
5. Click on the "Save" button
Images must be less than 3MB and will be automatically re-sized in the post. Click on the image to see it full sized.