First you find a picture on the web, take its url (the web adress of the pic generally starting
http://www...) create your thread and all, type your text when you want to add the picture click this icon
(located at the middle top of the box in which you type the text) and paste the url of the pic you chose, and save/click "ok"...
other than that you can also upload picture of yours to a picture hosting website (like tinypic, photobucket, picnik....and many more) then look for what is called the "img code" generally being presented that way:
[ img ]
http://www.... [ / img ] (without all the spaces).
all you have to do with this code is paste it directly into the text box, it will automatically appear as u valid/save the post...if it doesn't then it means there's either an error or the pic has been moved or erased...