And add to what TheOracle said:
If you want get proper support, do not do more and anything else but exactly what the guide told u to do
Ones, its installed, tested and everything works fine, then do whatever modifications u wish.
Now, back to your problem. I have no clue what could u do to categories.php or functions.php or other files that would affect $cat_id be always 0, so, test it on a fresh 4images, perhaps retrace your steps when did other modifications, etc.