What's truly scary is that some people actually believe the "honest businessman" in this scenario is in the right. Unfortunately, the schools are so busy teaching children "the freedom of information" that they forget to teach them "copyright" along with it.
There are things you cannot do legally, things you should not do ethically.
Legally, the copyright on not only text but also images, music, etc. belongs to the creators or anyone the creators sold that right to. In the case of music or images, even something hundreds of years old and "public domain" (like sheet music of Bach or a painting from an Impressionist painter) may not be free and clear, if you didn't record the performance of said piece or take the photograph of said art personally. In this case, even though Bach's sheet music is public domain, the individual recorded performance of a particular piece of Bach's music is copyright to the musician. SOME musicians will allow you to use said recording, as long as you give them attribution. Some won't. If they haven't given you permission, it is illegal...and actionable...for you to use that recording without permission.
With photographs you've taken personally, you're usually safe. After all, you own the copyright on it. I've been told that some localities have separate laws about photographs taken of what they consider "national treasures." I don't pretend to understand how they can claim copyright of a photo I took (hypothetically speaking), just because they own the original landmark, but I imagine it comes down to the same sort of laws that say I have to have permission of the person in a photo I took to use it on a book cover.
So, what is safe? If you want to use photos/videos/illustrations, go to a RELIABLE, ESTABLISHED stock photo site like canstock or istock, and pay for the correct license to use the photo you want. Same thing for music.
Alternately, ask permission of the copyright owner to use the work. Now, this is touchy. There have been actual cases where permission was obtained, and the person asking permission believed he/she had the copyright owner, but it was a scam artist. In those cases, the person who believed he/she had permission was fined harshly for the use, because it was the only way for the copyright owner to protect his/her copyright...going after everyone involved, even those who didn't know they were breaking laws and thought they'd done the right thing.
With written content, it's a little more cut and dried. Unless you're using a version that was updated/translated/Americanized/etc., it's easy to establish that (for instance) Shakespeare is public domain. As with recordings of music, a translation of an older piece or an updated/modernized version of an older work is copyright of the person who made the changes. Anything that remained unchanged is still public domain. Anything that has changed...off limits without permission.
In the case of authors and cover art, your contract should allow you reasonable use of the cover art (whether that contract is between publisher and author for art the publisher arranged for or author and artist for art the author commissioned for the book), which would include displaying the art on the author's website. Still, some copy centers demand proof of the contract before allowing the author to use his/her rights in regard to it to make promo gear.
Looking for the "perfect" picture to use on your site doesn't mean you have the proper legal rights to use it. That is something that all "honest businessmen" should understand.