Oh, if you really want to help photographers, then use Stockxpert. At StockXpert I get 2,5USD for the same (8MP) pics I get 0,25 at SS, 1USD at IS and 1USD at DT.
DT is tad cheaper, but a sale StockXpert is worth two at any of the other sites.
Apart form that, Istock has the best collection and DT surprised me with so many free pictures, so that's worth signing up for anyway.