Don't be bashful! If you want comments, give comments. You don't need to be an expert photographer to express your opinion about the work of others. You don't need to go into great depth. A simple remark about cropping, composition, color, subject matter, etc. can be of just as much help to a budding photographer as an in-depth critique.
Take one of your photos from an assignment in a previous chapter, and turn it into a black and white piece of art. Choose carefully so you get the best shadows and highlights — something that has a lot of contrast as a color photo will have the best results. However, ultimately, choose something that you like and post your before and after images to the Web site.