I don't know if discussing this here will help at all but I do hope at least some stock artists see this.
Ever since I started doing book covers for authors, I have had to consider one more thing to look for in the stock rules, available for commercial use or not. Some stock providers say it is OK, some say you have to ask them first and others don't say anything. In cases of background stock, there is always an option to look for other stock images. When it comes to models, the characters on the covers have to be very precise so finding the right model is very difficult. So once I find the right one, I try to work it with the model as much as I can. In my one year of doing this, I have had several instances where the stock artists never got back to me or did that very late (a month or more). Now you don't expect the client to wait for that long for you so these replies are really no good after so long. I have also had an instance with one of the biggest and best stock providers on dA