This makes sense in terms of eliminating random content, and more importantly, editable content. The most useful files are those you can customize for your project. However, PNGs can be useful from time to time. There are many times I'm looking for an image and I just wish the silhouette (cut out) work was already done for me. I think folio could be a good place for things like that too, and they may come in the form of pngs.

+1 on "loads of sites out there that do photography already and far better" - good move on sticking with the layered/interface elements