Make cropping square, not just straight lines that you have to re-align after moving one vertex. And make it possible to move the entire square/rectangle over the taken picture, such as when you've got the right info in there, but the selected area isn't quite right.
Eric Dunbar commented
This is an oversight on the part of the developers.
#1 Needs to be able to move the crop rectangle by pressing INSIDE the rectangle and dragging elsewhere.
#2 crop rectangle edges need to allow user to select edge and drag whole edge in a single direction. There is no need to add a square on the edge to clutter the interface, simply make the edge respond to the tap and drag.
Otherwise it seems like a good program.
Making the cropping square would make it inflexible for those situations where you're slightly at an angle. If adding this, MAKE IT OPTIONAL!