Now I might be wrong but that's the impression I have had for years.
Where I'm a little hazy is in my reading of 17 USC § 411(c) - I believe that you can only sue for copyright infringement within 3 months of the work being "first transmitted" (which I assume also includes publication), and 48 hours after the work has been "transmitted".
Not sure if there is some other part of the U.S. Code that applies though.
I'm not a lawyer, but I don't think 411c applies in this case. That subsection refers to works whose transmission and fixation are simultaneous, which I believe is specific to live broadcasts and performances. I'm not entirely clear on how all of the timing language there is supposed to work, but I don't think any of it applies to webcomics anyway.
What would apply, however, is section 412 below that, which basically says what I outlined above; if you want to sue for anything beyond actual proved damages, you have to have registered either before the infringement, or within 3 months of creating the work.
"Except for an action brought for a violation of the rights of the author under section 106A (a), and subject to the provisions of subsection (b), [1] no civil action for infringement of the copyright in any United States work shall be instituted until preregistration or registration of the copyright claim has been made in accordance with this title. "
Copyright protects your articles, illustrations, posts, photographs, videos, and other works of creative expression from the time that they are fixed in a tangible medium of expression. It gives the creator of an original work exclusive rights to the work, for a limited time.
That is the true definition of copyright, the second you put pen to paper. You do not need to register with a copyright office.
Copyright is automatic, but the right to actually litigate a copyright is dependent on manually registering a work. If you register prior to infringement, you are eligible for very generous "statutory damages", which are roughly $100k per act of infringement. If you register after the infringement, you are only eligible for the actual damages (i.e., lost sales or the defendant's ill-gotten earnings from the infringing uses) that you prove in court.