For module art in particular, it's pretty much locked to the module. That's because you're buying it as a "package deal" (i.e. the art may not be licensed for separate use outside that module). However, we are working with module creators to see if some would like to offer a bundle (e.g. a module + a token set of the art from the module). Finally, you can copy + paste the tokens/other art and paste it as much as you want inside that campaign, you can make new pages, etc. So you could start a new campaign (a copy of that module) and re-use the art in it even if you don't play through that module specifically, you just can't use that method to mix and match art from different modules.
