I am The Compendium
If I have a publisher, do I need an agent?
My experience is with novels and not graphic novels. I'm not sure how much difference there is, and it's not a yes or no question because it depends on a lot of other details, but here are some thoughts:
If you want to do your own submissions and negotiations, and if you've found a publisher on your own, you can certainly keep going that route. Nothing at all wrong with it, so the short answer is no, you don't
need one. And if you've accepted an offer, an agent wouldn't be able to do much for you at this point because you've agreed to the terms (but they could negotiate for future works).
Contracts that are offered to unagented writers are worse deals. Not because the publishers are jerks, but because they know they can offer less or refuse to negotiate much or at all from their boilerplate contract. You just don't have as much leverage (unless you have a prior sales history, or a big name, or some kind of huge platform, etc). You should also make sure to research contracts and make sure you know exactly what you're signing and what kind of clauses to look out for. If you decide to submit to other publishers, you'll still be limited to those that accept unagented submissions.
If you have an offer but haven't accepted (even verbally), there is a window to look for representation. The general way to go about this is to ask the publisher for a bit of time and to email agents with a clear "Offer from [Publisher]" in the subject line. If the publisher is a small press and/or pays little to no advance, most agents won't be interested, so that plays into things as well.
It's a personal decision, and one you should make after researching the benefits of an agent and what they do. Personally I think having an agent is awesome because they guide your career, know tons about the industry, know editors, know contracts, and are there to help with the business side of things. They can also negotiate to keep foreign rights and film rights and try to sell them, meaning you don't have to rely on whether the publisher will (and you will keep more of the money). But there are lots of writers who prefer to go it alone, and that's fine too. It comes down to what works for you.