As been said in this thread, here are the basic factors.
1) Where your traffic came from. feeding it circle jerk traffic? expect low figures.
Feeding it TGP traffic? Why send people who want to view ONLY THUMBS to a site they have to read? low figures
SE traffic = good
other blogs = good
targeted traffic from sites they had to READ at = great
bring in readers, make good sales. bring in freeloading pic skimmers, make bad sales.
simple