500 x 500 is just a round number. For 512 x 512 it will cost 51.20$.

50$ is just a round number for lunch and possibly tip.
I think the price is low for other types of images such as portraits, for reasons including that more "painted" or realistic images can take many many hours. Also because the price is fixed up to 500x500 pixels.
Like I was saying variations are available, that would include if a customer wants to buy exclusive ownership. I mainly just want to be able to later publish the image myself if I want to. For example if I had submitted an image to any of the games I've seen here, I wouldn't care if they published the game for profit. I just don't want my drawing showing up in a game where like the game play is about living the life of a child predator narco terrorist. Of course no one at shmup-dev will be making CPNT. I think not buying exclusive ownership will lower cost for an indie developer.
On that web page it's just meant to be an "online sketch book", and I might put up an online portfolio like your suggestion. I see your point about the portfolio. You could say that web page is currently informal and this offer is informal. You would advocate not making any offer in the future without a directly linked to portfolio?
Although there is no images there such as a game main menu or sprite sheet, I will make one for a customer that wants one.