image command replaces part of the original image with the image given as an argument. Since the image given as an argument has only a single channel, the part replaced in the original image is only replaced on one channel as well (in this case the luminance channel).
Here, you copy a smaller image into a larger image. But the smaller dimension is not in width or height (which are identical), but in number of channels. But the idea is the same: you copy a table of values into a larger table of values.
The call to
j  is actually equivalent to
j ,0,0,0,0 (as the default values for the other arguments are
0). If you want to copy your single-channel image in the
a channel for instance, rather than on the
L channel, then you would have to write