I wouldn't worry about it. There are plenty of other sites out there that host covers, and they often specifically target bootleggers, pirates, etc. (AllCDCovers.com and CDcovers.cc are just the tip of the iceberg when I do a Google Search.) If we were to suddenly become private or whatever, then the bootleggers would have plenty of alternatives (given, they won't be better alternatives, considering many of those types of sites rarely highlight the need for high res covers) while legitimate cover-ers would have to deal with red tape and road blocks just to do what they want to do.
Long story short, making the site private would be punishing everybody for the deeds of a few.