If you get artwork made from scratch, you should get a concept sketch at some point (at least I have from all the DeviantArt artists I have worked with over the years) for approval, which goes a long, long way to prove the finished art is the artist's own work.
If want an opinion from the other side (an illustrator) , I completely agree... yet though, I'm rarely (in my never book/writing related experience, probably similar to those at Deviant, though I never wanted to be a member of that site) asked for a sketch. Or at least, for that purpose of serving as a future proof. Also, as I'd have been sending so many PNGs for showing progress / requesting feedback for direction in some details, that usually the client might end up with so many internal snapshots that would be proof in any court about that author having got the original from the artist, and how is not based in anything else. Despite the healthy habit of signing both a contract in what is just image rights, etc, at the start of the project. Some of us are not too much into showing an early sketch. I sketch "functionally", not to make pretty/expressive line art. Also, they have my own "code", so, often the other person doesn't really know where I'm going (might if is also a painter) and can trash a very promising line of work. IMO the snapshots in the middle , as you draw, are more than enough. For proving someone is not using a catalog image / tracing something, or the horrid thing of copying someone else's work... all can be tricked... I have certain principles about that, and can't even feel good if not doing it all in my rigid way ( to the extent, I'd prefer to go back to code websites if having to do any tracing/copying!), but I can tell you I have seen many people tracing and then disguising it... :/ (this all speaking about full painting works. Design is a total different animal, tracing and using photo, textures, is the craft itself, is in its essence. Is all about composition, vibe, treatment, retouch...I know as I have done a ton of that, too, inside companies. Is a very different category, in my opinion. When I do that, I don't really paint, I retouch, over-paint. Is a totally different thing. )
I have had good experiences with artists on there, but as Denise says, they're not cover designers, so it helps if you're very specific about what you want.
Certainly...I'm not versed in anything about writing self-publishing world, but neither was in board games first times (which are almost always printed, is also print related stuff, and still need also the digital output (webs, KS campaigns, videos, etc)) , but typically authors know a lot about general habits, common places, and what they are after. Often extremely better than the artist, while the professional artist/designer knows the deal in composition, color, lighting, drawing, anatomy, etc. The more you can communicate, the better (same goes with the artist). Then there's also the great association between a designer who knows deeply the field, and illustrators... Better said, LOL, I have known that well in
other areas... Maybe here is a very rare case. No idea...
