how does iMessage work - is iMessage on iPhone free to other iPhones in canada and on telus ?

Im a little confused about iMessage and charges and fees from cell phone companies

if i am not mistaken iMessage means you can send text or pictures or videos to other iPhones with iMessage without being charged

i send and receive a lot or pics to and from my iPhone (with iMessage) to other friends iPhones who have i message as well and i am still getting charged on every bill $5-$8 for picture and video messages

why am i getting charged?
do i have to be in wifi? or do i have to let telus know ? has this happend to anyone else ?