I agree with Tim, you should try Quiver, this viewer is amazing.
You can choose a folder to view, and then browse all the pictures by thumbnails. You can also view pictures and keep a thumbnail bar on the side. The very nice thing is that you can load pretty large pictures : 10 Mpixels JPEGS from my Panasonic LX3 are quickly displayed, zooming and panning is fast ...

In my opinion, Quiver is the best picture viewer I've ever seen on a mobile device.