By graphic novel, I refer to illustrated texts supported by images and visual representation, not necessarily a comic book but rather one which relies on its imagery as much as it's text to convey itself.

That's sort of a difficult question to answer, if only because there have been a number of illustrated books but not always many in a style that quite fits your definition. If you want to get technical about it, the first work in an identifiable "comic book" format is Les Amours de Mr. Vieux Bois (1837) by Rudolph Töpffer, which was very popular and translated into many languages, printed and reprinted for years, but isn't quite a "novel" as such.

Closer to what I think you're getting at is the 1920s and 30s period, which saw the publishing and popularity - even critical esteem - for a number of wordless novels by the likes of Otto Nückel, Frans Masreel, Lynd Ward, and Milt Gross, such as God's Man (1929) and He Done Her Wrong: The Great American Novel and Not a Word in It — No Music, Too (1930).

These were all written, drawn, and published well before what we think of today as the "comic book" was invented, but in an era when newspaper comics were very popular and new artistic styles and methods of printing were being developed and popularized; Ward in particular pioneered the wordless novel in the United States, having been inspired by the earlier works of Nückel and Masreel; Gross was more of a cartoonist who saw was Ward was doing and turned his hand to parody - and in fact Gross wrote and drew cartoon-novels such as Dunt Esk (1927) with both words and pictures (Gross wrote in an exaggerated dialectal English), which I think matches the kind of description you're going for.

Actual "comic books" of a format familiar to today's readers pretty much started with Action Comics #1 in 1938 (although you can quibble about that date; there were separate magazine-collections of newspaper comics before that date).

Also, what would be an example of a prominent comic book, that gained mainstream attention during the time when comic books were relatively unknown or unheard of?

I would say the rise of the graphic novel as we know it today occurred with the publication of Art Spiegelman's Maus, which began serialization in 1981 and was first collected in 1986. It wasn't the first graphic novel - the term was coined by comics legend Will Eisner in 1978 for A Contract with God and Other Tenement Stories - but it did hit mainstream critical appeal of a type that comic books didn't achieve for years.