picoproj: make the -f option mandatory

Passing an image file name is now mandatory, before that the user could
be induced into thinking that a command like the following was sending
an image:

 ./picoproj -W 800 -H 480 -F 1 some_image.jpg

while the actually intended command line was:

  ./picoproj -W 800 -H 480 -F 1 -f some_image.jpg
                                ^^
Note the missing -f in the first command line.