Easy, it's null because the DOM element does not exist.
Now let's look for the error:
Which DOM element has been requested when the error occured?
Answer: the one with an id of "trailer1_info-toggle". Doesn't exist. Check.
Why has it been requested in the first place?
Have a look here:
It shouldn't have tested positive for /peli/, right?
document.getElementById(divobj.id+"-toggle").src=(state=="block")? "http://cineout.es/wp-content/themes/infimum/imgs/collapse.jpg" :
document.getElementById(divobj.id+"_info-toggle").src=(state=="block")? "http://cineout.es/wp-content/themes/infimum/imgs/collapse_info.jpg" : "http://cineout.es/wp-content/themes/infimum/imgs/expand_info.jpg"
Answer: It didn't. Your indentation suggest braces where there are none.
Edit: Darn you, Old Pedant! I should have just given the answer instead of writing up that fun little quiz.