Wot 'e said.
At the bottom right, next to the "Post Reply" button. It'll open a dialog box so that you can navigate to where the pic is on your computer. When you click "Open" the pic will be sent to the forum.
When the pic has uploaded, you'll get the opportunity at the bottom of the screen to leave it as a thumbnail or show the full image. If you click on the full image button the picture will show in the message.
If you want to upload more than one picture, do remember to press enter once or twice on your keyboard before you upload the next picture otherwise they'll go one next to the other in a long string and we'll all have to scroll sideways to see them.