think there might be a software out there that prevents a picture from being copied without having to use a water mark i saw a college classmate that had used it but never asked
Your easiest solution would be to use as Dan suggested a single pixel GIF, although there's no sure fire way to prevent pictures from being taken. If they can be viewed, they can be taken too.
disable right click, make a watermark and also resize the image. If its small thats better than full size imo. Also keep the quality wont hurt as people want to see your work at its best. I dont believe you could prevent a screen print but theres 1 exception. I know you cant save a dvd format using screen print. Since this is on the web, I dont think that will be an option.