The argument X should be a list of pixel images
(objects of class "im"). If the images do not have
identical pixel grids, they will be converted to a common
grid using harmonise.im.
At each pixel location, the values of the images in X
at that pixel will be extracted as a vector. The function
FUN will be applied to this vector. The result (which should be
a single value) becomes the pixel value of the resulting image.