How do filter 2-D image only using 'filter' function?

if I don't use 'filter2', or 'imfilter' function, How do filter 2D image, or array? should I use 'for' loop? In this case, I want to find average of 5 continuous elements by using 'filter' function

example) original image : http://www.cosy.sbg.ac.at/~pmeerw/Watermarking/lena_color.gif

using filter -> inserted image

Please anwer me~~~