#compdef -P (p[bgpn]m*|*top[bgpn]m)

local expl

if [[ "$words[1]" = pnm* ]]; then
  pat='*.(#i)p[bgp]m'
elif [[ "$words[1]" = *top[bgpn]m ]]; then
  pat="*.(#i)${words[1]%%top[bgpn]m}"
else
  pat="*.(#i)${words[1][1,3]}"
fi

if (( $# )); then
  expl=( "$@" )
else
  _description expl 'picture file'
fi

_path_files "$expl[@]" -g "$pat" ||
    _files "$expl[@]" -g '*.(#i)p[bgp]m'