This is what they used for its theatrical release:
http://upload.wikimedia.org/wikipedia/en/c/c4/Name_of_rose_movieposter.jpg
Sheesh! No wonder people ignored the movie...
Personally I love the poster and the art style and even have it hanging up in my living room next to my front door. However I do agree that it does not fit the spirit of the movie in the slightest bit. It makes it look more like a whimsical fantasy movie than a gritty and disturbing intellectual "whodunit".
There were several learning-to-read comic books from Fisher Price for kids in the 1980's which came with cassette tapes to play to listen along to while reading the books. I had both "Robin Hood" and "Treasure Island" and both of them I remember had the exact same art style on the cover with the characters in bubbles on both sides of the page. I wouldn't be surprised if they were done by the same artist or were heavily inspired by each other.
https://thumbs.worthpoint.com/zoom/images2/1/0515/06/vintage-fisher-price-classics-comics_1_7ae7a31e3d6821c44faed92c798fc80f.jpg
https://thisoldtoy.com/new-images/images-ok/3000-4000s/FP3100-EB71203348-B.JPG