Derder, When you add a post either
a) click/mark "Preview reply" on the left in the "options" box or
b) click "quote on your right in the grey line above the post you wanna reply to.
Then your WRITE your answer, THEN click "CONTINUE" in the bottom, THEN u get a chance to add a picture from a file on your PC.
Simply press "choose file" (or whatever it says in that button in your language) and then navigate to the drive and folder where your file/picture is stored, press CHOOSE, and VOILA!
Remember to keep the pics below 300 K.

Hope this helps.