These superheroes are trademarked property and the holders of those trademarks (DC, Marvel and such) are very protective of their characters. The person who made the original iron-on's is required to pay a licensing fee to the mark owner for the right to use the superheoe's identity, characteristics, look, colors, etc. You can not re-create these characters, put them on to merchandise and sell them unless you too obtain a license from the mark owners.
As for modifying the characters; Trademark protects characters to the extent that they are recognized. If someone recognizes it as Batman, then that is enough for infringement (even if you changed it some.)
As for purchased iron-on's on your personal clothes; you have paid for the iron-on, therefore you have paid for the right to wear an apron, t-shirt with that character on it.
But... Trademark law protects the mark owner against a "likelihood of confusion" as to the source of the product, in this case, aprons. To buy 5, 10 , or 1000 iron-on's and put them onto aprons and sell them, would lead the consumer to think he was not just buying a homemade apron with a legitimate iron-on, but an entire decorative apron that was manufactured by or approved by DC or Marvel. (Just as buying official NFL patches and then putting them onto hats and selling them would be trademark infringement.)
Bottom line, the consumers could think they were buying aprons (or NFL hats) that were made by or approved by the owner of the trademark (superhero or NFL) and that is trademark infringement. I would urge you to see an intellectual property attorney to prevent Marvel comics from knocking on your door.
My disclaimer is simply that Avvo already has an adequate disclaimer.
It all depends on the exact use of material you end up doing, but speaking generally, you cannot sell merchandize with those logos ironed on without infringing copyright and trademark. Changing them slightly is not enough. You will need to acquire a license from the companies that actually own the trademarks and copyrights (not just any old "licensed" material) that specifically authorizes you to sell branded merchandize. Good luck!
Please note: this answer is intended to inform and educate. It should not be taken as legal advice or a legal recommendation, as that would require a thorough review of all facts and circumstances. You may discuss the possibility of hiring me as a lawyer, but until we have an agreement in writing, there is no attorney-client relationship between us. Thank you, and good luck!
No, this is dangerous. These are famous trademarks. You could use the logo on your own shirt for personal use as that is not "in commerce", but once you sell you are "in commerce" and you are likely infringing both trademarks and copyrights. You might be small enough to go unnoticed, but don't bet your business livelihood on it. Trademark and copyright infringement can be a one-way ticket to bankruptcy due to the potential for enhanced damages. Before doing this you need to see an IP attorney for help in getting licensed, and a license is unlikely unless you are doing a large business or have something that miraculously attracts the copyright owner to license. This is likely a non-starter.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.