Yeah, that's a "lol no" from me, dog. I would never, ever respect a writer who went and stole someone else's art. If you're not an artistically-inclined writer, imagine this... someone just goes and peruses through your words, pulling out whole sentences and paragraphs and maybe even chapters, and then says that "well it describes my character/world/plot the exact way I would, so I'm just gon' go ahead and use it."
The thing too is you can often contact an artist and ASK them if you can use it for a free webnovel. I imagine at least sometimes they might even say yes if you give them credit, or offer you a chance to commission them for your very own cover for your precious baby of a novel. And for photographs it even more common, and they would probably say yes more often or charge you a very modest fee for use of their work. Like... there's no excuse.