There are two things you need to consider, particularly when it comes to graphics, logos, and super hero names (plus things like popular phrases).
The first is copyright - this pretty much automatically applies to all created works, no registration etc of the work is required, and by default the creator of the work is the copyright holder unless they pass the rights to someone else. You need to seek permission to use any work that is protected by copyright, especially as you intend to use it for commercial purposes (that generally requires a license, and fees).
Some work is in what is called the "public domain". That means the copyright on it has either expired, or it was released into the public domain by the copyright holder. An example would be old artwork. Some content is released under what is called a Creative Commons license, which has a variety of levels - some CC licenses allow you to just use the content but not modify it and not for commercial purposes, others allow you to use / modify it including for commercial purposes, but with proper accreditation of the creator of the work, others still allow you to do whatever you want with it, without any accreditation, so if you find something that has a CC license, be sure to check what type of license it's been released under.
The second is trademarks. Trademarks are a little different to copyright, and apply to a different kind of content, like names, logos / symbols, phrases etc, and they don't just cover the exact rendition of the trademark, but anything that is similar enough to be able to be likened to the original (so, you can't slightly alter a trademarked symbol or logo, put it on a mug and sell it, as that would be a breach and leave you open for financial penalties).
You would probably be surprised at just what has been trademarked these days, so research your designs very carefully, because others have managed to trademark even ancient symbols like runes and some commmon every day phrases.
Some companies will sell licenses to their content, but it's usually a very involved and expensive process, especially for the more well-known stuff.
