No. Many of the pictures in the photo gallery are not portraits. The rating bar is for users to register votes for thier favourite (or least favourite) pictures on a sliding scale for the purpose of the Top Rated views of the picture gallery and the random pictures on the homepage, which are collected daily from the top 500 "worksafe" pictures. The only purpose of the ratings is for you to let me know what pictures you like and to make it easy to find the very best pictures in the gallery. The computation that assesses the votes is more than just a simple average like you would get at a rating site. For example, a picture with several 4-star ratings will score higher in the ranking than an image getting one 5-star rating.
Now that there are often hundreds of photos from a given event, the ratings are also used to automatically decide which ones to keep in the permanent archive. Photos that have not been rated above a certain threshold are expired after three months.
For the mathematically inclined, this is the formula used to rank the pictures. N is the set of responses in the range of one or two stars (which have a negative value) and P is the set of responses in the range three to five stars (which have a positive value.)