Comparison between 2 images in Frequency domain
1st question: there are some simple steps to change the spatial domain to freq domain, exp adding some zero to avoid ISI, you can read the procedure from a source book, you can use fft2 command in matlab

2nd question: Surely you can not see the differences in freq domain as clear as spatial domain, because to determine the value of one pixel in freq domain all the pixels in the spatial domain will be effective