The pictures can either be on another website or just on your hard drive.
Click "Go Advanced" at the bottom of your post.
Scroll down to "Manage Attachments". Notice that your picture has to be in one of the formats listed. When you click the "Manage Attachments" button it will tell you how big the picture can be.
If the picture is on a website, type the URL on the line and click "upload."
If the picture is on your computer, click "Browse", find the picture (I often move them to the desktop to make them easy to find) and click on the name, and then click "Upload."
It should show the pictures name in the window when it is done. Then click "save", and then back in your original window, click "Submit Reply."