Is there any Mac Application used for part-zooming a picture?
This feature is built in ML 10.8 with the accessibility feature zoom. Go to system preferences Accessibility, select zoom the last feature is zoom style select picture-in-picture. This will be a rectangle and follows your mouse any where on screen. You can use option command = (to enlarge) or - (to decrees) the text under the magnifying rectangle, if you have a keyboard with num pad you can use option command + or - for the same effect.