Obviously, the origins of the comic book owe much to MANY different sources, and it's all but impossible to value the degree to which one source affected comics' development over another. But one source that I've long felt was overlooked was the Art Nouveau movement that was popular from around 1890-1905. It was at a time when printing became cost-effective enough to use it as a cheap promotional tool, so artists trying to make living began adapting Art Nouveau techniques to produce more work faster. And one of the methods of doing that was by eliminating excessive cross-hatching and shading that typified printed illustrations and simplifying figures and forms to their outlines.
I first made the connection while reading ElfQuest back in the mid-1980s. Wendy Pini's Glider tribe, and Winnowill in particular, reminded me of Aubrey Beardsley's style from almost a century earlier...
Because of the reduced linework in Art Nouveau -- despite all the extra flowery bits often put into the background -- it was easier to crank out work quickly and many commercial artists adopted the style to earn more money. Indeed, the "classic" artists that are trotted out during Art Nouveau discussions -- Alphons Mucha, Aubrey Beardsley, Henri de Toulouse-Lautrec, etc. -- were commercial artists producing works for advertising works for companies. Many of their pieces shown in the art history books are ads!
I recently stumbled across the work of one Alberto Della Valle. I can find very little information about him online, but he was apparently a commercial illustrator in the late 1800s and early 1900s working out of Italy. Most of the works of his I've found are book cover illustrations and, while not particularly historically significant in light of Art Nouveau pioneers, still seem well executed in and of themselves. That these covers were largely for pulp novels ties back in with Steranko's ideas on the origins of comics. Stories about pirates and heroes and adventurers, coupled with simplified line illustrations seems to make for an obvious bridge between pulps and comics.
Della Valle seems to have been a fairly small-time artist, who's main notoriety comes from having illustrated the covers to several Emilio Salgari novels. If anyone has any more information on Della Valle, I'd certainly be interested to hear it.
And with that said, I'll leave you with a small collection of Della Valle covers. Take a look at them and see if you can't easily imagine these pictures leading to whole comic book stories.