Nudes, etc., are permitted as avatars. Later on, they'll be censored from profiles, because those will be G-rated. The photo will still be on your site, as the front page.
To put an image in critique is really easy when you're uploading them. You select critique or not (default is not) in the edit window where you put in all the info about the photo.
If you want to put it in later, you go to "Your Portfolios" and select the proper port. Click add/edit images. Select the image you want in critique and click "edit image." Then, you'll be on the proper page. Go toward the bottom, and you'll see the option for critique.