You need to change your general settings to WYSIWYG. On the turquoise menu bar at the top of your screen click "Forum Actions". Then click General settings. Scroll all the way to the bottom of the page under Miscellaneous Options and click the button for "Enhanced Interface Full WYSIWYG editing". Click the blue save settings box at the bottom. Like magic you will see the little tree icon for inserting images.
Edited to add your tree looks very nice.