time for i in *.bmp; do convert "$i" "$i.png"; done

time sh -c 'for i in *.bmp; do convert "$i" "$i.png"; done'

time is only defined for Simple Commands by POSIX. Timing loops, command groups and similar is not.

None. If you use a shell that supports this (e.g. bash, ksh), specify this shell in the shebang.