Macadamia Apps has GroupShot (iTunes Link) for the iPhone, iPod Touch, iPad in the iTunes App Store for FREE ($1.99 value) for a limited time. This deal expires March 11, 2012.

GroupShot solves a common problem found in group photos, namely one person who isn’t facing the camera or whose expression doesn’t fit the rest of the group.
Usually people take several shots of group photos, with the hope that one will be perfect.
With GroupShot, users can pick and choose parts of each photo, then combine all the parts to create the perfect shot that looks exactly the same.

The usage is incredibly simple. All you need to do is to load your images, mark the parts you wish to replace, and choose the best option from the rest of the set.