Source: Big Light
As a hardcore X-Files fan (yes, after all these years I still love the show), when I watched the show back in the 90s, I have always noticed that Mulder’s “I Want to Believe” Poster changed in season four of the show (probably because I’m the only one geeky enough to notice such a small detail). So, I decided to contact Frank Spotnitz, one of the main writers/producers of the show, to find clarity on the issue.
Here’s what Spotnitz had to say:
“My recollection is that there was a problem with the legal rights to the photograph used in the original ‘I Want To Believe’ poster. It was resolved by changing the image.”
Not a big deal that this change was made, but since there was never an “in universe” explanation for the change, I have been curious for years as to why it happened. The reason why I noticed this change was because this poster is one of the most iconic and memorable things about the show. You can forget every other detail about Mulder’s basement office, but everyone remembers the “I Want To Believe” UFO poster hanging up on his wall. It really embodied who Mulder was in one simple image.
It is nice to have closure on this matter. I can finally sleep at night. 
vinicius sobieski said
October 20 2010
hi, ‘im nuts too…I also noticed the change of the poster, because I am very detail oriented, what could be bad or good, still do not know .. anyway the copyright of the original poster belong to whom? the picture is real? or simply an illustration?best regards from Brazil
Spyder Virus said
October 24 2010
Well, since it gets torched in “The End”, he had to get a new one, and through some manipulation of The Space-Time Continuum, the new one ended up in the past as well! It’s all there in Case No. X-S5E20……LMAO! Sorry, couldn’t pass up the opportunity!
But in all seriousness, glad to see I wasn’t the only one who noticed and thanks for the enlightenment as to why! 
Jeroen said
January 12 2011
The left one has way better designed typography, the right one has a more interesting picture. Which one was the original?
Dan Geer said
January 12 2011
The left is the original, and the right one first appeared in Season 4 of the show (in the episode, “Tunguska” I think)