You used to be able to click a photo and pull up the original. Now they have switched to the slide show format. So what happens is that when you upload a photo, Hubpages makes two versions of it. One is for the hub and will be either 520 or 260 pixels wide, depending on if you are using it for the full hub width or half. The second copy is 1024 pixels wide that they use in the slideshow. If the uploaded original is smaller than 1024 pixels than they use the 520 or 260 pixel photo in the slideshow.
If you for instance click on a hub photo to save it, the file name will end with "_f520.jpg" or "_f260.jpg". A slideshow photo will end with "_f1024.jpg".
This is just based on my own observations. if I'm wrong, hopefully someone will correct me.