OK. I would *keep* Template.php, also obviously Settings.php and Settings_bak.php. Also obviously keep anything that came in the SMF package

YaBBImages can go *only* if a few things are true. Firstly, if you converted to existing yabbse template the current template may point to the YaBBImages directory so if you delete it you may have to manually go and change the template.
Secondly, all your users avatars are probably pointing to the avatars sub directory of YaBBImages so that will most likely need to stay...