Chele is correct.
Depending on how you have your general settings set (found in Settings) you may or may not be able to easily post a pic in Quick Reply. If you do not have a bar at the top of your Quick reply to change Fonts, Size or add image and wish to have that, then go to Settings at the top of the page, choose General Setting (on left hand side) and at the bottom of the page in General Settings is a heading named Miscellaneous Options. Choose the first option "Enhanced Interface - Full WYSIWYG Editing" and save changes at the bottom of the page. You will now have the bar and quick access to your images and editing options in Quick Reply.