Make sure you read, in full, the license for the stock image that you'd want to buy. Are you allowed to modify the image, like change the color? Are you allowed to put it on a product that you're then going to sell? What, if any, credit do you need to give? Any sort of "copyright" concerns depends on the image in question and what sort of license you're paying for.
As for AI, it's been established that anything generated using AI cannot be copyrighted, as something has to be made by a human in order to be copyrighted. You might have heard of the monkey selfie copyright thing awhile ago and that case established that a human has to make the art in question. Many stock image sites do not allow AI generated images on there, as you need to have the copyright of something in order to sell it to someone else, so, legally, the image maker doesn't have the "right" to sell the image. And there's nothing stopping anyone else from using it. Many image generating AI thingies also say in their terms of service that you can't use what it spits out for commercial purposes...like selling the image to a stock image company. It is, simply put, an absolutely legal clusterfuck. Disregarding the morals of using AI, it's not something that you as a small-pocketbook'd individual want to be involved with. You don't want that legal risk.
Anyways, tons of trade publishers utilize stock image when they create their covers, so it's not a weird thing to do. One of the editions for a Sarah J Maas book is using an AI generate stock image, actually, and people found out and they're very mad. And it's the publisher for Harry Potter, so it's not like they don't have the money. Using an AI generated image puts you at risk of people thinking poorly of you and since your cover is trying to convince people to give you money, it's a risky thing to do!
