A mixed bag of interaction and expectation...
You are using the photo app on the iP(hone/od/ad) you know there are a few familiar gestures to interact.
Tap to select an image.
Swipe left or right to go back or forth through the album.
Pinch or stretch to zoom in or out.
On any level of zoom one or two fingers navigates the plane of the zoomed image.
These sets of gestures are transferable in other apps, i.e. Cooliris, Flickr et al
What gestures would you imagine, given the current operation of gestures on an Apple multitouch device would flip an image horizontally or vertically?
I am looking to employ gestures that do not conflict with the current implementation of gestures featured in the above apps, so whilst keeping one touch opposing the direction of another, (Think scrolling using touch pads on the Macbooks. A solution James is tackling in his answer) might seem perfectly reasonable to us here it doesn't translate to the iP(hone/od/ad) device as it just sees a pinch.
I have found one gesture that doesn't mimic the others and its swiping up or down. In fact this does nothing unles you are zoomed in.