WDADU established the career of Dream Theater, of course.
Much like the notion that "prog began with King Crimson" doesn't really hold up to scrutiny if you have a knowledge of the music scene of the late sixties, "prog metal began with Images & Words" doesn't either. I&W is basically to prog metal what In The Court Of The Crimson King was to prog; the album where the genre became commercially acceptable. Fates Warning's Perfect Symmetry came out
three years earlier than I&W and is demonstrably a prog metal album. (If you ask me, it should be on the poster instead of WDADU, as they both came out in 1989, but Perfect Symmetry tends to be massively overlooked in the evolution of prog metal, as is Fates Warning.) I think it's fair to put WDADU on the poster because it's the band's first album, and at that time, progressive metal didn't exist. If you were around back then, any band with a remotely progressive tinge to their sound "sounded like Rush."
I&W is the seminal, classic, quintessential prog metal album, but it isn't the first or the one that established the new song for prog. Just the one the world noticed first.