Hi Erik, if the images you have are of the exactly same location,but have differences due to satellite calibration,earth rotation etc (that is they are of different dates) after having chosen your GPCs you select warp displayed band from the option menu in the GPCs window, then you choose the warping method you want (eg,polynomial etc),the degrees of freedom, the resambling method (eg Nearest Neighbouring) and the output filename and its location in your pc.