These are outdated screenshots (from version 1.1)
In the upper half, the current image. In the lower half, unscaled parts of the image where the shifting will be calculated. The slider selects the index of the image currently shown.
The images are loaded into the programme, then automatically aligned using 2 specified reference points (this allows for a small shift, rotation or scaling between images, to be corrected), then merged (average).
The images are aligned by means of a cross correlation algorithm on two zones. These "reference" zones are chosen by the user on the first image. The use of the excellent fftw library allows nearly instantaneous correlation calculation for 200x200 px reference zones (N log N algorithm).
The GUI is implemented using Qt4.