When writing fanfiction, like the current Comics Retcon Universe stuff, it's polite and considerate to acknowledge the current copyright and/or trademark holders, when known. Especially if using artwork or logos.

Just a line near the top or bottom of a story, or on the Title page is enough. And it isn't required here, it's just a nice thing to do for the original owners of the stuff.

Now, comics get a little complicated. Like who owns the copyright to Superman? It may surprise you to find out that it isn't DC comics, nor Time-Warner that owns DC. It's the heirs of Jerry Siegel and Joe Schuster. But DC Comics owns the trademark. Joe Simon and the heirs of Jack Kirby are currently on the way to getting the copyright for Captain America back from Marvel. And who owns Batman is a damn good guess, traditionally invented by Bob Kane the situation is actually more complicated.

Anyway, a note as I say is courteous but not required. People who write and post fanfiction here are taking responsibility for their own actions, I'm talking about courtesy, not legal niceties.

Of course, if any copyright or trademark owners complain, stuff will have to be taken down. That's pretty unlikely, though, especially in the realm of comics since both Marvel and DC recognize the value, to them, of having a lively fanfiction scene on the internet.

Hugs to all,
Erin