Comments can be added to a product image, an inspiration photo or to the project as a whole. For a product or inspiration photo, click the desired image, add your comment or question in the field labeled Discuss This Image/Inspiration, then click Post Comment. For comments about the project as a whole, use the Discussion field as the bottom of the project page.
In all cases, you can choose to make your comment private – visible by only select users – and attach supporting files.