Please bear with me, as I find much copyright law completely confusing, and what constitutes 'fair use' and allowed fan art and the like.

I'm doing a series of maps for my own homebrew (no 'official publication or money making intended), based fairly completely on existing prior art (say a computer game). If I make examples of those maps available online, like in a forum like this - is that against copyright rules? Do I need to say that it's based on the original XYZ game published by ABC Co? Am I even allowed to post such things?

I don't want to break any rules, but I also appreciate the constructive criticism that I can get from a place like this.

I looked for similar discussions here, but couldn't find anything beyond the discussion about people copyrighting their own work.

Thanks.
Neurowiz