I've lifted quite a few textures I liked from other people's maps, and tweaked and fiddle with them to get them just the way I wanted them. I've put quite a lot of work into many of them - at what point is the work not the property of the original artist?

Right now I'm doing maps just for fun, and to share with other D&D players, but a LOT of what I do is emulating other people's styles (to mesh better with the official maps from settings I work on). At some point someone may ask me for brushes, vectors, textures, etc... and I don't think I can really give-out stuff I modified, can I?

Aside from that, I'm afraid to post some of the stuff I've done here, because I used 'lifted' textures, rather then draw my own (I'm just not that great an artists). For instance, my current project is to extend an existing map (from the core D&D setting), which means the center of the map was actually made by someone else, in it's entirety. I'm not sure what the rules are regarding stuff like that.