okay so like, I'm kind of just molten rn bc this whole issue of exploitation of underpaid artist labor is why my hands don't work anymore, so I might edit this out later when I'm more sane, but…
so okay, the way art on independent sites like this works is that they rely on amateur artists in general. $15/hour is something like bare minimum living wage and since art is a highly physical labor, $30/hour is fairly standard base. $60/hour is if you actually get employed by someone who wants you to stay remotely healthy.
as a general rule with pet sites, they don't operate with a high enough profit margin for this sort of thing, so some of the things they do are either get gig work from amateur artists and pay them a flat rate for the item, or offer the users a way to generate content on their own. Both of those rely on the tradeoff of having “off-brand” or “unprofessional” art in your game, since you're basically looking for people who will accept unlivable wages.
that being said, sites often allow trading through currencies and offer players a way to “earn” money with their artwork as an incentive in some sort of elaborate ponzi scheme which is honestly fun to partake in, but full of questionable ethical choices.
Beyond that, there's the question of “how much leeway” do the sites give the volunteer artists to match the style and yeah! sites can be as strict or lax as they choose. The site provides the service of hosting the images so it's only fair that they can choose what they want to host and what they don't! but when you're already on the slippery slo pe of below minimum wage work, I think people have earned a degree of sympathy for not wanting to put in the extra unpaid hour or two required to check all 12 images they've donated to the site!