I think there's no "simple" answer to your questions.

About the drawing and the writer: I think the picture, the art is property of both: the writer and the artist. The picture or piece of art as it is, as it exists, couldn't be created by only the writer or only the artist.

The question about your mods: If you feel you should get credit for it and one needs to have permission to use it, you should let it protected by an organism (in Belgium we have SABAM, you go there with your work and ask them a, well, let's call it an license. You pay a small amount of money and voilà, your work is protected, nobody is allowed to copy it without permission).

I always feel its kinda contradictory if somebody makes, let's say a guide, and on the one hand makes it publicly accessible, for free, and on the other hand claims it's copy protected and you can't use it without permission of the author. If you put something for free on the internet, you can't hope nobody will use it without your permission. That's just a stupid (or better: naive) idea.

Just a few thoughts, hopefully somebody specialised in intellectual property rights will look at your post and will post a more professional opinion, so all those fine modders (respect guys! most be hard work!) here will actually learn something valuable.