I’m not sure on the legality of an informal contract like this but if they give permission it should be fine to use.
On the flip side with no legal contracts they are always free to ask you stop using it down the line and you’re also not covered if they’ve perhaps taken the artwork from someone else. I think this is unlikely but worth considering.
I’ve used artwork with permission before and so far have had no issues but the most I’ve done with them is use them for Soundcloud / Youtube thumbnails.
As far as I can tell deviant art and artstation is a total mix of amateurs and professionals so the response is probably going to vary. I’m guessing it will be the same as letting someone use your music, if they’re looking to make money from it then its standard to expect payment.