Copy an image to my website?

[Kyriakos]

Hello guys, i own a website that writes reviews about movies that get to post a picture in each post. I was searching in all the internet but nobody could give me a clear solution on what should i do to be legal. I just want to post the cover picture of the movie, which the companies create so its like i am advertising them for free. I know that there are copyrights but the thing i don't get, is how all this sites got the same picture and still is illegal. Do i need to contact the companies and ask for their permission ? and if its that how do i do it.

Thanks in advance.