In English, you have two distinct terms to talk about comic-strip style books: the term "comics" refers to a shorter, periodical form, whereas "graphic novel" is a longer narrative, often hard-cover "comic-style" book.

 

In French, we don't have two distinct terms as in English, so in both cases (comics and graphic novels), we usually use the generic term of "une bande-dessinée / une BD", fans going for the English term "un comics" for comics.

 

As for "roman illustré", that's a completely different genre of books, i.e. a written novel with illustrations.

 

I hope that's helpful!

Bonne journée !