I need some very specific art for my book cover and will probably not be able to find a generic one on a website, so do I hire someone for it and what's the average hourly wage for hiring one? How do I make sure he's the one who gets into legal trouble instead of me if he does something wrong and make sure I get what I asked for?
I've had very good luck with fiverr.com (yes, two r's). It is called "fiverr" because the artists are supposed to be able to do some (relatively small) thing for $5 US. I have zero financial interest in it, and I only recommend it because for me it has worked great.
I've gotten over twenty pieces of art there, from about four different artists.
You can see their finished projects online, and you can read their reviews, often alongside the art. You have to give them permission to use the art in your review, and they can't prevent people from posting a harsh review. (Well if they offer to refund you your money and you take it; then your bad review and rating would disappear.)
The site rules make it very difficult to get a refund if they deliver art. It is possible (I have done it when I contracted for some video-game music), but you have to make a very good case that what was delivered is NOT what you asked them to create. I was able to do that, I had very specific requirements in my instructions, about two out of six were met.
You have to negotiate a price, you won't get anything decent for $5, except perhaps a pencil sketch without revision. That said, these people are generally out of America and do not ask for much at all.
For original art, always look at their previous work to see if you like the style and quality they can deliver. Some artists clearly stick to cartoons, not realism. If you can, provide them pics of the style you like, they will tell you up front if they can match it, and they do not usually lie, they don't want a refused contract any more than you do. This is an online conversation through the fiverr website (they are not supposed to work outside the website, if they offer to do that, shut the door and do not use the artist! All communications are stored and can be reviewed, and you can be banned from the site for agreeing to that (or even giving the artist your email or phone number), and your artist may resent your refusal and then do a lesser job for being forced to pay his site commission).
I usually contract for $15 to provide me with a rough pencil sketch of what I have in mind; with two free revisions. I tell them if I like it, then we will negotiate the next step.
For reference, I got 16 slides for a fund-raising presentation. 2 were a building sketched from photos we provided with requested modifications, 1 was a kind of chart, and 13 of them had realistic illustrations of people (i.e. not at all cartoonish, which is likely what you want for a book cover); the people were from various occupations engaged in work activities. I paid an average of $90 per slide. In color. In another case, I paid for a video-game cover, again realistic illustrations of people, and paid $250; that also had realistic looking buildings, cars, landscape, etc. I've used fiverr several times for other business art, logos, product sketches, and so on. Added: You can also tip them, I usually do if I intend to use the same artist for more pieces (e.g. for a presentation, you want the same artist to do all of your slides, for consistency.)
The more complex the picture the more you will pay, obviously. Added: A "complex" picture means more characters or specific elements that must be included; so it may look simple but isn't. More characters, or scifi ships you want to look right, or guns or devices you want to look right -- The more control you exert over the content and composition, the more it will cost. Put all that control into the project description for commissioning the full artwork, that is what the artist will price. Control is also why I begin with rough sketches, if I use up my revisions I can even order a second round if I think the artist has nearly got what I want.
The gig price is negotiated up front; make sure you get full rights to the art. The artist cannot ask for more. If they are late, you can cancel the gig and pay nothing, or allow them more time (and still reserve the right to cancel after that expires). But if you are doing something like a Romance Novel cover (two people in love on a backdrop with a suggestion of story setting) I would personally not expect to pay more than $300 US.
Make sure your artist writes decent English, if you cannot communicate with them, you will not get what you want, and you'll end up with a dispute. This is why I start with a cheap gig for a pencil sketch; because I can give up $15 on a lost cause and try a different artist. Provide comparables (existing covers) if you can, and be detailed in what you ask for to begin the gig. If somebody doesn't want to do the pencil sketch first, that has happened to me. Thank them for their time and move on. Remember every message you send them gets recorded, the site will ban users for being abusive. If you have to argue about the work you contracted, be civil about it.
If the work is going to be a major piece, you can do three gigs for three stages: Pencil sketch first (with two revisions), then detailed outline work (with two revisions) (line drawings like you see in a coloring book), then colorization and texture (make the bricks look like bricks instead of blank rectangles, etc).
Good luck. I have personally been very happy with them, it is the go-to place for all my artwork, in business or writing.
There are sites where artists who do book covers list themselves as looking for work. And there are individual websites or social media for various artists. Look around and find someone you like, or at least get a sense of the style you're looking for. No artist can duplicate every style, though some are better at it than others. Your ideal is an artist whose usual style is the one you want.
The going rate for a book cover, with full art (pencils, ink, colors, lettering), is in the $100-200 US range, though an individual artist may charge more or less for this. It is a per page rate, not an hourly rate.
Do not skimp! A good cover is worth every penny because that is what potential readers will see, whether it is a printed copy in a store (or at your table in a sales room) or the graphic at Amazon or another online site.
Always have a written contact with your artist, even if it's a friend (maybe especially if it's a friend). There are templates around. You want to spell out the artist's rights, your rights, all the financial situations, where the art can be used without paying more, and about revisions. It is typical to pay all or some upon acceptance of the finished art.