To upload a pic from your computer:
Click on Go Advanced next to the Post Quick Reply
The message box will change and you will see a little paper clip above the white box. Click on it and a new window will open. Click on Choose File and find your file. Click Upload. Repeat for all your pics and then close the window and post as normal.
To post a pic from another website like Photobucket:
Click on the little postcard icon above the message box and copy/paste the URL of the pic into that.