Make sure you're in the "advanced editor" mode (automatic if you're starting a new topic, an option if you're replying/adding to a previous post) The giveaway is there are emoticons to choose on the left of the screen in "advanced", none in the "quick reply".
Click on the "attach image" box at the bottom of the page. This should automatically open a window that allows you to choose the file (in this case picture) you want to add. Double click on the pic you want and wait while the computer attaches it to your post. It will the give you the option of adding it to the text box if you want a full size pic.
