Thanks for that link. Now that I have looked at it he does say that a) he didn't create them and b) they were collected from the cartographers guild. I think links would have been better but links into this site don't show images unless your logged in.
When you upload an image here its yours. You own copyright to the image which means that you cant legally save a copy of the image to your HDD or use it without permission. Now nobody here is that fussed that they don't want it shared to some extent. Most people don't mind taking the image and printing it for your game and using it personally.
As creator of the image you have the rights to license your image or sell it or disclaim all copyright and make it public domain.
If you sell the copyright then you don't own it any more. Sometimes that's what happens when your commissioned to make a piece of art. Somebody buys the art and rights to it.
Most of the time however you license it and state under what terms you want people to use your artwork. You can make up any license you want but if your prepared to fight for your rights then it has to be something that a lawyer will defend which is not easy. So the "creative commons" or CC license was made where by you can take that license and apply it to your work. It comes in a variety of flavours where you can opt for some or none of the features. The usual is BY-SA-NC which means:
BY - you must state who made the artwork
SA - Share Alike, if you repost a derivative work of the art then you must do so using a similiar CC license to this one
NC - Non Commercial. You cant charge for its publication.
If you go down the public domain route then you give up rights. Anyone can do anything they like with your art as its not your art any more. This is the best artwork to embed into your maps as your completely free to do so. Note tho that history has shown that big corps who can afford lawyers might try to state that the art was in fact theirs all along and its then your job to defend the fact that it is in fact free again. Daft tho this is, that's how its become.
So most people make a statement about how they want their art used in a license terms that ought to be part of the image but often is just stated by the original poster of it.
So from my point of view. If anyone wants to use my art and its CC then as long as you abide by the BY-SA-NC then go right ahead. No complaints from me. This poster in gleemax has not abided by it because many of the maps are not CC at all and for those that were, he didn't post who made them. That's why it ought to be part of the image. Most people are too lazy.
You don't have to make a license but I recommend that you do. If you don't then by default you get the tightest possible - All rights reserved - anyway so its not something that means you lose your work by posting here. Note however that if you post to MySpace et al then you loose that right since by joining, your agreeing to give it all up to the owners of it when you post there !