I'm thinking about extracting all the buildings and other terrain objects of my copy of HOMM4, but if I do that and make a map with them, I won't be able to post it (legally) anywhere, right?
I just had a good look around the site and if I read it right, you can do what you want with them (maybe not commercially) as it clearly says in the about section that:

"This project aims to create an archive of clip art that can be used for free for any use.
All graphics submitted to the project should be placed into the Public Domain according to the statement by the Creative Commons."

So you should be OK.

p.s. I think your Realms maps are awesome. I also really love the new style introduced in 4E by Mike Schley